Passion Flower Refuses to Bloom

I purchased this vine-like plant-herb at least three-four years ago at your store-greenhouse.
It has not flowered once. The plant gets full sun, I've added to the soil a slow-release fertilizer and it does not get over watered.
What am I doing wrong?

Your fertilizer may be the problem. Make sure you do not use a high nitrogen one, because this will just encourage green growth, but no flowers.
A high middle number on the fertilizer you use will signal to the plant that a change is coming from whatever type of growth it was making. A high last number will signal to the plant that it is time to become mature. Maturity means flowering.
So use a high phosphate fertilizer (middle number) for a month, then switch to a high last number – potash (K) – fertilizer and keep using it until the plant sets flower buds. Most slow release fertilizers cannot be regulated this way and I therefore cannot recommend them. I have used a slow release fertilizer formulated for hanging baskets and it worked like a charm. It had low N, but high K.