Is fall to late to transplant house plants to larger pots,

Sure you can transplant to larger pots.
I bet you have over fertilized them! Young plants need minimum nutes till a root system develops. Too much water can drown young roots. Put a clear cover over the tender new plants to slow evaporation & prevent temp & moisture swings.
If too bright sunlight, screen to reduce light.

They have to be planted in Acid Soil not ordinary potting compost also watered with rainwater NOT Tap water. Feeding should also be for ACID so please check. Very important.

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Try and pot on the new plants into small pots filled with potting compost.

Let's pretend it's an artificial plant!

The normal way to remove a plant from a pot is to hake sure the potting compost is slightly damp, then, holding the plant's stem between thumb and fingers, with the palm flat on the compost, to invert the pot. Now tap the pot gently, and the plant should slide out of the pot without damage to either. If the plant is pot-bound to the extent that roots come out of the central hole, trim these off beforehand.

When re-potting, ALWAYS pot up just one size. Spread the roots before re-potting, and make sure you use new compost.

It may need more room for its roots / a bigger pot with fresh potting soil for additional nutrients.

Growing Requirements for Rex Begonia Plants Rex Begonias grow best in bright, indirect sunlight coming from an east or west window.
Fluorescent grow lights are also an excellent alternative to natural light for these plants,
and in many cases will intensify the foliage colors.
When potting, set the plant with the crown slightly above the soil line.
They should be planted in a porous, slightly acidic planting mix that contains some peat moss
and leaf mold. African Violet potting mixes work well for Rex Begonias.

Allow the top 1inch of soil to dry before watering, then water thoroughly.
Over watering can lead to root or crown rot! Wet foliage can cause problems with mildew.
Always use room temperature water when watering house plants.
House plants should never be allowed to stand in water for more than a few hours.
Painted-Leaf Begonia plants thrive with daytime temperatures of 70-75
and 60-65 at night. NEVER lower than 60!
They require a humidity level of at least 50 percent and appreciate periodic, light misting.
Provide additional humidity by setting the plants on shallow trays filled with moistened pebbles,
or a humidifier, especially during the winter months.

Feed actively growing plants in early spring and again in early summer with a slow-release plant food,
or every 2 weeks using a quarter strength dilution of a soluble, complete fertilizer (23-19-14).
Decrease feeding in early fall and stop altogether in the winter. Do not over-fertilize!

Your Begonia Rex may go dormant in the winter and possibly drop its leaves.
If this happens, continue to water sparingly until spring when new leaves will re-appear.
If your Rex is growing where it only gets light from one side ( a window sill),
turn the pot a quarter turn each week to maintain even growth.
Foliage color may fade under low light conditions. Consider using a grow light.

Remove any dead leaves and other debris around plant regularly.
Rex Begonias enjoy spending the summer in a shaded part of the garden.
Always be sure to check for insects and pests before bringing your plants back indoors!
Rex Begonias are susceptible to attacks from mealy bugs.

It could be that the container they are in is too big. The top layers of soil are the first to dry up so if a container is too large for the plants, the nutrients and water could be washed down beyond the smaller plants reach. I would start by having the soil tested at a local plant and tree nursery. Over time minerals and salts can build up in soil to levels that may be tolerable for a larger plant that has grown in that soil it\'s entire life, but will mean doom and death to smaller plants or different types of plants. If the soil tests ok,. then I\'d try a smaller container. I\'m no expert, this is just from my own experience. Hope it helps.

How long have you had the tree?
Indoors or out?
If indoors, do you have any new plants in the house?
Have you moved the plant recently?

Your problem could be any one of:

1. An infestation of tiny insects. Spray a solution of soap and water all over the plant, top and underside of leaves, until it is dripping. This will kill the insects.
2. Overwatering. Hard to fix. Try transferring to a bigger pot and mixing in new dry soil. Or poke holes in the soil around the roots with the handle of a wooden spoon to let the air in.
3. Underwatering. Put the plant in the bath, turn on the shower, set to tepid (i.e. not freezing cold and definitely not hot) and give it a good soaking. But take the pot out of the tray or dish and let it drain. It should not be in standing water. Don't water "little and often" - this is bad... give it a good soaking and then let it dry out.
4. Chemical build up from tap water. If you see a salty-looking depsoit aorund the pot, or brown depsoits. do the same procedure as above.

Hope this helps.