Tempermental Chainsaw

I have a Stihl chainsaw, model MS310, 59 cc engine, that I bought new five years ago. I cut down 6 or 7 trees each year and saw them into firewood len gths. Other than an occasional fallen limb after a storm, that's all the us e it gets.
A couple of weeks ago, I felled a couple of trees and started sawing them i nto logs. About halfway through the second tree, I shut off the saw to move some limbs or logs from underfoot. When I tried to start it again, it woul dn't start; felt like there was little or no compression when I pulled the cord.
Last weekend, I went out and cut down four trees without any saw problems, it restarted easily when hot.
Today, I went out to saw up the trees from last weekend. After cutting the first tree into logs, I went over to the second tree and the saw wouldn't r estart. Again, it felt like there was no compression; there had been normal compression resistance when I first started it. I checked the fuel and bar oil tanks and they were both about half full.
Any ideas what this could be caused by? I checked the spark arrester screen and it was clean. Air filter is clean and new. Spark plug is ok, fuel/oil mix is what I've been using all along. I have several chains and keep them well sharpened, rotating the use among them, so the engine isn't being stra ined by a dull blade. I couldn't find any internal areas packed with sawdus t which would cause the engine to overheat.
Any ideas of what else to try before taking it into the shop?
Paul
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mine is always pressurized after it has run.
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