You holding notes a bit too long . IF you in game and you hear the same sound for like 30 seconds it will get irritating .
Make sure you utilize silence a bit . With ambient music sometimes you want to have pause instead of just holding noise entire time .
Also even though you don't need melody in ambient music you need to strive for something that people who didn't make your song can pick up on . If pads are too long it may be difficult to tell what theme of music is .
Good examples of ambient music can be found in SIlent HIll and Dino Crysis. Even ambient music has some kind of rhythm so you don't want to just hold pads all the time .