Polyurethane "cures" in two stages:
1) The thinner (mineral spirits) evaporates away (known as flashing off)

2) The varnish polymerizes by absorbing and reacting with oxygen in the air.

#1 happens in a few hours, #2 happens in a few weeks

How long was it before you used it?

In addition, really heavy coats skin over and prevent oxygen from being able to get to the inner parts. You can have a run that is gooey for a long time.

An easy way to do poly finishes is with a wiping version.

Here is a link that might be useful: primer on wiping varnish