When you are pressing seams in heavy fabrics, it helps to use a clapper.  The fabric goes on the seam roll with the wrong side up, then the seam gets finger pressed open with a moist rag or tailor’s dauber, then…

Whammo.

If it is a really heavy fabric, make sure to use some sustained pressure after the initial strike and hold it.  Be careful about overdoing it with this, though.  You want to snap the fibers into shape, not beat them into overworked oblivion.