When a plant isn't doing well under the "right conditions" regarding light, water, temperature, it's almost always an issue with the roots (usually in combination with poor soil that is almost always peat with little else added to increase tilth and moisture/AIR movement.)

Here's a Begonia/Tradescantia combo pot long overdue for repotting (as apparent by the decline in flowering). After trimming off some excess Begonia branches and about half of the length of the Tradescantia, it's ready to investigate inside the pot:

{{gwi:93732}}

{{gwi:92441}}

Chopping off the bottom half with a shovel reveals - nothing:

{{gwi:92442}}

No roots on the inside and evidence of potting up, not repotting:

{{gwi:92443}}

Here's another Tradescantia still in store-bought basket-bound condition (more about this at the end) from last year, covered with buds that quit opening:

{{gwi:93465}}

Roots with nowhere to go:

{{gwi:93466}}

Another case of no roots in the center of the "root ball" which is really a root sphere on this plant:

{{gwi:93467}}

Compared to the roots of a plant grown in better soil, a cutting from the mama Begonia ('castaway') shown above:

{{gwi:12003}}

I filled this pot with cuttings about 3 months ago. It really didn't need to be repotted but I wanted to see and show (show & tell?) what's going on in the pot.

{{gwi:93733}}

{{gwi:93734}}

The roots are not all on the outside but they don't show up well in this phone pic:

{{gwi:93735}}

Now when one puts home-made yard pile compost in potted plants, it's necessary to expect some things (critters) to be in the pot sometimes, but it's still kind of creepy and I'm not immune to flailing backwards and screaming like a little girl if something moves and I don't know what it is (and sometimes when I do.) But this little guy or gal really SURPRISED me!

{{gwi:93736}}

I am so glad he/she wasn't at the level where I chopped with the shovel!

{{gwi:93737}}

I think I'm forgiven for the disturbance and was rewarded with an introduction.

{{gwi:93738}}

Then we found a more suitable home in a moist shady corner where I keep a little dish of water for the lizards, where I hope this cute little critter decides to stay and eat as many bugs as his little gut will hold. If he eats ONE mosquito, he's my hero!

{{gwi:93739}}

Anyway, I'm glad I decided to repot that plant just for demonstration because these chunks were not in the soil, just kind of hovering above, and most still a ways from extending their roots far enough to get to the soil. Amazing, I guess the water from the hose was enough for them (and now I see one of the things that might have disturbed them from how I had them, with at least an end of each in the soil.) Think I'll stick these in the ground where I put the toad since he seems to like it.

{{gwi:93740}}

Store-bought basket-bound condition... These plastic baskets are plant-killers by design. Not saying that's the intention, but the result is what it is. All water in the ring-around-the-hole is unable to drain from the pot. That plastic thing at the bottom does not keep the roots from making contact with this water, which can cause rot, and takes up space that could be filled with roots/soil. The most critical thing to do is make some holes in the true bottom of the pot. The easiest way for me is to use a sturdy pruner to nip some little triangle holes around the outer rim or the inner rim near the hole.

Then there's no reason not to remove that plastic thing and put more soil. The increased root space combined with better drainage can do wonders especially if it's a plant that has stopped blooming prematurely.