I had that problem when I bought some lipos that used Deans knock off plugs instead of the real thing. I just replaced the plugs on the battery packs with real deans plugs from the LHS and all was good. I'll bet this is your issue.
I can't decide if its more fun
to make it...
or break it...
They are real Deans plugs I put them on myself. I remember them being very tight in the beginning. However now they are not. They actually came off about 4 times when I was running in about 12 minutes of runtime. Both sides were coming out. Would not be good on a track that is for sure.