Clients who are first time punters often have this issue with me, think it's the whole intimidation/pretty naked girl thing and they usually say oh I'm nervous etc. I often find the key is to distract them, usually laying down next to them and just kissing really passionately for a while means they get hard because they're not thinking about it and it's not even being touched. If that doesn't work I'll whip out my toys and give them a little show and get them involved. Usually the ego boost about helping me orgasm helps with the erection issue. Sometimes it can be helpful to put a condom on when soft and letting it get hard IN the condom so there's no issue of them losing it.