We have 31 kegs. Most of them will not seal unless we put pressure on the. Sometimes it takes a lot of pressure (20+) If we need a lower pressure, we just dump the co2 as once sealed, they normally stay sealed.
You also need to get some food grade lubicant. We use it on the rubber and it helps the seal. also, we have had to readjust the seal to get it to seat. Onething you will notice when you have beer in them is that the beer will absorb the CO2. If you think you have a keg pressurised at 10 psi on top of beer (or water), and come back in a couple days, the CO2 is gone. It did not leak but was absorbed.
I normally soak with oxiclean or sun oxgen ( I like the sun product better) overnight. I have an extra keg that I use for cleaning and shoot oxiclean solution through both side(beer our and Gas In) with a hose set up for just cleaning. We can ladder beers with this many kegs, some for a year.