Crib sturdyness

Question

I have a crib that I got used from someone that had it for only one child. After assembly, it seemed to rock back and forth like it wasn't all that sturdy. It still works however, but I was wondering if I was missing anything during assembly that would cause it to do this. The mattress spring frame sits on some brackets that are screwed into the ends. The crib front bars go up and down within their slots like they're supposed to. So what's up? Does anyone else have this problem?Thanks in advance..