Pentax K-r auto focussing problem

I bought a K-r about 3 months ago. after a while, I found a bad problem in its auto focussing in low light(indoor and outdoor). I really confused, it can't focus in these conditions. Does it have any solution or I should sell it? I had an Olympus DSLR before but it never had this problem. (of course it never had any problem). please guide me in this matter.