GT 5000 Series Having problem with push rod.?

I have a craftsman GT 5000 Series. The exhaust push rod broke on 1 side 2 weeks ago. I replaced it and regapped both intake and exhaust valves. I mowed the lawn ( 1 acre) 3 times after that, then the same thing happened. Same push rod same side. Can anyone please tell me why this has happened or what to do so that this does not keep happening?