Hey Clive, along with different wood, you might want to check that your blade is all the way seated in the bottom clamp also. I had a similar problem until I realized that the non-cutting part of the blade was coming up through the table and into the wood, which means the blade is not cutting at that point but just pushing on the wood on the upstroke. Now ever time I change a blade, I make sure that the arm is in full upstroke and that only teeth come up through the table, before I tighten the bottom clamp. This does seem to be a bigger problem with the reverse blades, at least it was for me. I also cut off about a 1/4" off the bottom of the blade with a pair of wire cutters to make sure the blade sits a little lower.
Bob