It depends. This depends on the individual. This may be because of too much stress. It might be from a medical condition, like diabetes type ii, or low testosterone. This may need counseling, like with a psychiatrist or urologist. Self help and supervised group therapy can also help.
Normal. You're not supposed to be hard after ejaculating. That's the resolution phase. You need to wait some time (maybe even an hour or two) before going again.
Unsure of question. I'm not sure of your question. If you do not lose your erection after ejaculation will you experience lack of pleasure? Erections that are unwanted painful and lasting hours need urgent medical attention.