don't know about craftsman, but mccullough, and home light, have a tightening dog or screw that when the bar bolts are loosen, screwing the tension screw will cause the bar to move out alittle to apply tension to the chain. When installing the new chain and inserting the bar into the proper alignment and position, the "dog" or burr must line up in the square hole on the bar so that it will function. Sometimes people will tighten the bar up to the saw not aligning the dog up and the tension screw will not work because the burr or dog is not in the hole.
Also the tension dog will not work if the bolts are not loosened so that the bar will actually micro move to tighten the chain.
A bar will only go out if bowed or the end cogs are not turning around any more, or if the chain channel is compromised.
Take yours apart carefully and clean it thoroughly and look for the dog and the appropriate hole. If the bar is up side down the dog and hole will not meet.