In an official forum post, Blizzard confirmed that fixes are coming to Reinhardt’s hammer in the upcoming 1.12 patch, with these changes currently available to test in the PTR.
The trouble with Reinhardt’s hammer stemmed from the last patch which nerfed his Earthshatter ability. Players are now finding that when they swing his hammer, damage from each swing isn't taking effect immediately - but only registering when the swing has been completed. This means that - for now - Reinhardt is inconsistent when it comes to dealing damage at close quarters, making him far trickier to play effectively.
Note that these changes won’t take effect until the next patch has gone live, as Blizzard is encouraging feedback on how his hammer feels in the PTR first. When it does hit the live servers, it should be the quality of life change needed to bring him firmly back into the competitive fold.