We have hot water radiators in our older home with a boiler powered by electricity. Two of the rads are really hot the others seem to be normal, I have bled them. One of the rads that is really hot does not hiss or run water when I opened the valve to bleed it. What's wrong?

Keep bleeding it until steady stream of water comes out. Go to the highest point in the system and start there. This can take some time.

OK what I have now realized is that water does not seem to be reaching my upstairs rads, there are three of them and when I open the valve, nothing comes out. I have stood there with a valve open and after the hiss there is nothing even after 10 min

OK. Go to the last one that is hot and purge it for a long time until you have a steady flow of water and keep going for several minutes after. And work your way up in series. They should work eventually.

Could it be something to do with my psi because it is the upstairs ones that are the problem and my psi is at 8, shouldn't it be 12 or so

Has it worked before at 8 psi? Do you have a filter in line? That can cause pressure drop if clogged. You should get water out the other side of the rads that are hot. Do you have a bypass valve to shut off the up stairs? It may be closed.

Another trick is to hold the bleeder valve under water as you bleed it and watch for bubbles. You may have to use tubing to do that and or shop vac it.

See I don't have the details like age and how often you have it serviced if ever. But it could even be a blockage in the line. I would call a Tech if I were you if what I said does not work. A blockage could be the whole problem or even a heating element if you have more then one.