The feed dogs might be getting worn. I had an old Singer that happened to. I could have taken it to get new dogs but instead I made a cardboard gasket. This only works if the feed dogs have a screw that lets you remove them. I cut the gasket from thin cardboard the same shape as the bottom of the feed dogs. I then put it under the feed dogs and screwed them back into place. It lifted the dogs up just enough to advance the fabric well again.
Or the humidity is making the machine bed sticky? Give it a good coat of polish and see if that helps.