2015 Mercury 115 outboard power trim problems.

Trim up and down works fine while at rest. As soon as i get underway i can trim up fine with no problems, But i cannot trim back down while underway. The problem started a few weeks ago, but then it seemed to of fixed it self and worked fine for a few weeks. Now today it started acting up again. I checked all the wire connections going to the power trim and theyall seem snug. I'm kind of hoping its a easy fix, it will be covered under warrenty but my boats on a lake that you can only access with a ATV so its a pain to get in and out.