I believe I figured it out.
It's connected to one of the ads on the page, not sure which one.
If the ad doesn't load, because the website it comes from is down or lagging or whatever, the job doesn't pay.
I noticed today that I had just two red frames instead of the ads at the top and bottom of the screen when the job didn't pay, while the next job where there were ads to see did pay.
Maybe it's just a display issue (page rendering). I'm not sure jamboom would make an ad display to have an impact on backend (database) functionality ...
Well, today I even found a workaround for the problem.
I just look at the jobs page and if there is no ad at the bottom, I refresh the page until an ad appears.
It's a pain, but that way the jobs always pay.