I had a similar problem with my Evinrude trim.... Like 77 Viper said, the problem with mine was the solenoid. To test, I simply swapped the two solenoids. As soon as I did that, it would trim down but not trim up (the opposite as before the swap). Good luck

Welcome to the forums.

If there are two wires going into the trim pump motor, one blue and one green the one will be power and the other ground depending on witch way it was going positive to blue for up and green for down. If it's a three wire the black is ground and the blue, green are positive.
First check for power going to the motor in both directions. If there is power going up but not down it's most likely a bad solenoid.

hey guys i wanna start off by saying hi and you guys have helped me with a lot so far with everything.. I have a 84' viper with a small 100 merc I just got it in the water yesterday it ran great, and today I got in the boat put the trim all the way down got on it and trimmed up the boat ran great again but when i went to trim back down it did not do anything it only wants to go up. i played with wires and all but i dont think it a loose wire maybe an air lock? please help thank you guys