The breaker box is testing fine until I plug something in.?

I am working on an old building. The new breaker box is testing just fine. I have 120 on both legs all the way down the panel. When I attach a plug to the breaker and put a tester in the plug it lights up correct. When I then plug a lamp into that plug, I get nothing. The leg without the breaker now tests 0, and the leg with the breaker tests 240 but the breaker is not being thrown and the breaker itself tests 0. Below that breaker the leg still tests 240. What on earth is going on?