pentax k50

i have a Pentax k50 some times when i am Taking pics it dus not focus just keeps making a sound like a motor going in and out i put a post on a Pentax page some ware can not find the email it said take it back i got it from Jessop they said 25 pound to send it away even as it was bort in dec 15 ? as i can not make it do the problem message resent to Pentax on this page

All cameras may fail to focus on occasion. Things like poor light or a low contrast subject can be a problem. Also trying to focus too close.
Is the problem repeatable? If so take it back to Jessops with the receipt and they'll either help you sort out the problem or get it repaired for you (presumably you have at least a 12 month guarantee.)
Without seeing the camera and the way you're using it it's hard to say what the problem might be from a forum post, that's why it's best to get someone else to look at it for you.
Just say to Jessops you bought it from them last December (they'll have a copy of the receipt on record if you've lost it) and say you suspect the camera might have a focussing problem; do they agree?
If it works, it's persumably OK, if it doesn't then they have the proof they need to get it fixed for you.
Anyhow, welcome to the Pentax User forum and hope you succeed in getting it sorted out.

Have you tried different lenses? I have trouble with the 55-300mm lens sometimes when it hunts back and forth trying to find focus.