Take it back and tell them what's wrong.
I had a chain problem in the front on my new bike. VERY intermittent. Took it back within 30 days of my purchase and they couldn't find anything. Rode it for another few months; and by now, I was getting REALLY gunshy about not being able to get the gears to engage when going from the middle chain ring to the climbing gear (it would slip or LOCK UP!!)
I took it back in after doing a couple metric centuries. This time they were very careful and looked harder. They decided that the rear derailleur hanger was slightly bent. They gently suggested that i could have done it by dropping the bike. (sorry; i haven't dropped the bike, and it's been like this from the get go)
they fixed it free and I paid for a new chain.

and now it's ALL BETTER

so get back there and tell them.