standing erection

I did not take propecia but I took accutane. From what I gathered, they have somewhat similar side effects. My situation is somewhat strange:

-I do get nocturnal erections

-Difficult to maintain erections during the day (sitting) but not impossible

-Impossible to maintain erections standing. There is also very little/no sensation while standing.

I have heard of this inability to achieve standing erections as a side effect before on this forum and on others. I was just curious if anyone had any idea why this is? Does it have something to do with hormone levels?