It's commonly called Badass Jobbing Syndrome.  
 

after hunter prey doosmday he was still stronger he was in superman wars an his power still showed it was after that the bad writing start

Are you referring to Doomsday Rex where he was beaten rather easily by Superman, I think he had his DNA mixed with Superman's that weakened him

I believe (along with what others have mentioned) also it was revealed that Supes had been holding back in their more famous encounter. So both maybe a lot more stronger. Its hard to tell without a reliable, accurate, point of reference, 

Doomsday lost strength after H/P because the writers knew there would just simply be nowhere for him to go after that if they just kept him getting more and more powerful.  After all, with the power of reactive adaptation at his proposal, he could theoretically challenge almost all adversaries and then defeat them.  Where does one draw the line? The Presence? The Voice? The Overmonitor?  He had to be drummed down thereafter in much the same way Silver Age Superman needed to be depowered as well.  Can't have unbeatable characters in comics..you just can't.

they had to make him lose strength really when u too powerful only a hand full can defeat u so power him down only makes sense that way u can write him back as many times as you want