Old Skilsaw model 77 blade won't tighten
Some things to consider.


Some of the newest blade, especially the thin cut carbide tipped ones, are quite a bit thinner than the blades from 20 or 30 years ago. It is possible that the truss washers on either side of the blade just do not tighten down enough on a super thin blade. 
You may have made a mistake assembling the blade mount. The truss washer on at least the outside end of the shaft usually has a flatted hole on one side so that it fits down on the flat on the saw shaft. It is possible you had this rotated around the wrong way and the bolt went tight but the washer did not seat tight down to the blade.
Maybe there is debris or rusted threads down in the bottom of the threaded hole on the end of the saw shaft. This could prevent the bolt from entering the shaft the required amount to tighten up the blade.