Leaking bathroom pipe?

We recently replaced our pedestal sink with a sink and a vanity. The old pipe fitting under the sink were steel. Since the new sink sat further from the wall than the old sink, we had to replace the discharge pipes. I replaced all the fittings under the sink with pvc including the pipe that went into the wall. However, the pipe that went into the wall is now leaking at the wall, not at a connection. I'm not sure what the fittings look like once they're in the wall, but the new pvc pipe is in as far as it can go. Does anyone know what I have done to cause the sink to leak? Is there anything I should have done to replace the pipe that goes into the wall? Should I put a steel pipe back in its place, etc? Thanks!