I've found post op it does nothing without manipulation and was the same when I temporarily tried tablets, Cialis 20mg.
It also seems the more you do it the better it gets and the more desire there is. The less you do it the worse it all gets. I find if I don't do it I could probably never do it again but usually there is something telling me get it going even if I feel like I can't be bothered. Then suddenly it's a goer.
It also works best when standing upright or near upright. Lying down it does very little. In reality it's temperental but is getting quite a bit better. After two and a half years.
If you've had one small erection there is hope and with time it should improve. Tablets, vac pumps and working at it should help. If it is mental then perhaps there is something for that.