Since you said you're careful about eliminating pockets in your strips, my guess would be one of the following:

1.  The papier mache isn't completely dry.

2.  The paper is sticking to the mold a bit, so that, when you pull it out, its distending areas of the mask into pockets.  Some mold release of some kind might solve that problem.

Hello, and welcome to the forums!

I've had the same problem, and it is one of the reasons I'm not thrilled with PVA glue.  I've even done FLAT work, where I could press firmly all over, dried it thoroughly, cut through it, and found those unglued gaps.

But let me put forth a few possibilities...

1) If you are working quickly or the PVA glue hasn't been thinned 50/50 with water (although some cheap glues are 'pre-thinned' and watery, probably no need to thin those), bubbles may form on the surface; then, when two layers of paper are put together, the bubble bursts, leaving a spot on the paper that is not covered. Another relatively remote possibility is that certain sections of the paper have been coated more thinly and have started to dry before the next layer of paper is applied.  These don't seem likely to me, but I'm just tossing them out there. 

2) Your wool paper sounds extremely interesting, and I would like to hear more about it.  Wool is naturally coated with lanolin, an oil.  If all of the lanolin has not been removed from the fibers, I wonder if it is causing a 'greasy' spot that the glue is not sticking to properly?

3) If you are coating your molds with Vasoline/petroleum jelly, is there any chance at all that you are transferring some of that to the paper, via fingers or tools?

4) Are there particular spots on your masks where this is happening repeatedly?  If so, try burnishing those areas particularly well, possibly with the back of a spoon (a baby spoon for small spots).  A woman I knew eons ago had a clever husband with power tools who took old thrift shop spoons and ground them down to a series of graduating sizes for her, just for burnishing papier mache.

So, dry glue, bare paper, or grease are the only things that I can think of that would be a cause.  Unless the problem lies solely with the PVA glue itself, which is plastic-based.

Please post more info about the wool paper if you have the time.

And it would be wonderful if you posted some photos of your masks in our gallery.  Those Venetian masks are BEAUTIFUL!

Sue