I’ll go out on a limb and guarantee that you aren’t lifting the tip of the bar while you’re tensioning the chain and tightening the bar nut. Every saw manual out there specifies to do it that way. Do an experiment: tension the chain with the bar just hanging in the air, leave the bar nut loose, then lift the tip. The chain will get loose. You want to tension it in that tip lifted, chain loose position, because that’s the only position the bar will remain while you run the saw. I do it by setting the tip of the bar on a log, bench, whatever, hang onto the rear handle with one hand, use the scrench with the other.
+1 on putting a little pressure on the bar when tensioning.
+1 on checking the drive sprocket, a worn spur will be very hard to correctly tension if the chain starts riding in and out of the wear grooves