It's very difficult for GPSes to operate under heavy cover.

That said, the newer SiRF based units are MUCH better than the previous versions. My eTrex Vista is an old model, and my 60CSx is a new SiRF based model. The 60CSx is so much better it isn't even funny.

On the older units, typically you can find a hole in the canopy (watch the sat locater display) and stand still for a long time and you should be able to get a lock. The old units pretty much won't lock while on the move. Sometimes once you get a lock it will keep it while you move, but probably not, esp, on a quad.

-john