If you're talking about Justicia brandegeana, then I'd say that now is not a good time to judge it. I grow it too (in a big pot now, but I used to grow it close to the house foundation in the ground) and it looks very puny this time of year. Loss of leaves, certainly.

For the rest of the year it needs adequate sun and good fertilizer to do well. I don't know if growing it in sandy soil is affecting it; neither of mine grow in sandy soil. In that situation, nutrients would wash away faster.

Certainly would expect that the shady location is a major part of the problem.

Where I live you can only grow these in pots and they have to come inside way before first frost. I have been able to root cuttings to overwinter them but my babies never took off the next year. It became easier to just buy new plants whenever they started to look sad. They do like rich soil with lots of fertilizer and regular watering. I can usually keep them happy for two years before having to replace them.

It can be done - I have had mine in a pot for 10 years now. Last year, I cut it back to the soil level except for the best couple of stems. This year it was the most full and beautiful ever.

Every November I drag it into the garage, every late February it comes back out.