Every single crepe myrtle I have seen, has been outdoors. I'm in the UK, where the climate is temperate, but even here, they only ever go - and stay - outside.

Are they common as indoor plants where you are?

My guess i that it's just the wrong condition to keep it in. A kitchen is highly variable in temperatures, and right by a window, subject to variable lighting at this time of year, I actually think you may be killing it with kindness... Put it outdoors, clip it back on the opposite side to reduce the ball size to that of a tennis ball, and let it adapt to outdoor living.

Would be my take.

" Crapes like sun! If your tree is receiving less than 8 hours then expect to see less blooms than those out in full blazing sun. The more the better!

Wild fluctuations in temperatures during spring can play havoc on when your Crape breaks dormancy. Some varieties are known to produce early buds, which if hit by a late frost will set them back a few more weeks. Alternately, extended summer heat increases your bloom time greatly. Crapes are native to Asia and love to bask in the sun and cook in the heat. Haven't you ever wondered why those trees growing in parking lots look so amazing. Its all to do with radiant heat baby!"

From here.