So, these 20 plants have been kept going since last fall--having brought them in before being touched by frost; they've been put into a sunny window---and pretty soon, the sun values have dwindled to the point the plants have been given hardly any sun at all---such is winter---and the plants cant possibly keep growing like they do in summer.
They get gangly....the stems are weak....the leaves don't have the greeness they usually do and the whole plant suffers.
You cant feed them...they're not growing anything to feed.
You water them as much as they use it...which isn't much..since they are not putting on new foliage.
The bloom takes a lot out of the plant and the plant loses vigor.

Question: What do we usually do to push a plant to grow new leaves?............I think you know!

Cut it back. The roots will have less to feed and will begin growing new leaves.
Give it good light......south or west exposure is ideal.
East if you have to........and north just wont do...
do what you must but give it more, increased sun.
Don't fertilize....until new foliage is well along.
Then feed about every third or fourth watering with 20/20/20 liquid fertilizer.
Water it well ---to the point it drains out the bottom...dump the excess....and allow the plant to dry down between waterings.

Pretty soon new leaves will emerge and within a couple of weeks, it should be well along on its way to good health.

May I suggest next year, instead of keeping the plants going all winter....that you allow ten of them to just be put into a cold room....a coldcellar...or anywhere they can be put so as to not freeze...and let them sleep the winter away dry as a bone...never being allowed any water at all...put in darkness, never being disturbed...until
NOW...this time next year...mid February, when you bring them out, cut them back, repot them, water them and give them the best window you can.
They'll reward you with increased vigor of bloom and growth.

OK, so you don't have those 10 plants giving you bloom thru part of the winter.....big deal! You get them back in the spring...bigger and better than ever before.

Around this time of year I cut back my pelargoniums and use the cuttings to make more pelargoniums. They are very easy to root - they don't need alot of humidity like rose cuttings and such. I just stick the cuttings in good quality potting soil and put the pots in a window. Around planting time in May these cuttings are as big and blooming as those in the store.

I've got plants I have kept going for 4 years now. About now they do get gangly looking. Mine are under 400w HPS lights so light isn't the problem, they are blooming also. Every year, about this time, I cut them back to about 4-6" high. No, there won't be any leaves left but they'll soon regrow new growth and look good again. Take the nicest tip cuttings that show nice green growth that doesn't look woody, the cuttings will be the end pieces only and be only about 3-4" usually. Knock of all leaves except 2 or 3 smaller ones at the very top of each cutting. I like to use a soil-less mix to start my cuttings, keep damp but never wet. That'll get you more plants to keep or give to friends.

Don't be afraid to cut back those plants!

Indeed it can be scary to drastically cut back a plant. To give you some extra courage, think of how many other plants are pruned by man or nature. Many perennials die back to roots every winter and yet they regrow fine. Many bushes that grow long stems from the ground get cut back to 6-12" every year and regrow.

Some plants won't survive such drastic pruning, others will. Geraniums will.

can a geranium grow from only a leaf cutting? or an entire stem? i never tried them and need details, please