Taking this as an example:
The atmosphere is good, it's just a bit thin. I know you want to keep it simple, but two instruments don't keep interest very well unless they're playing more than one note each (the pad might be, I can't tell) or have a lot of effects added to them. The bell also feels quite...haphazard? As in, it's tracing out a melody that doesn't have much of a rhythm and doesn't play with engagement level very much. Most melodies rise, fall, speed up, slow down and pause to build tension. You've got some of that, but maybe not enough. There's no section where the bell stops playing for longer than a second, for instance. It doesn't have much of a recognisable motif running through it either. You don't always need that, especially for ambient music, but it helps.
It's hard, I'm having trouble putting my finger on why as well. The audio quality isn't bad. The instruments could use more effects (see if they have any settings you can tweak, and have a go at changing them for the entire track or even during the track). Really just experiment with all the tools you have available, and try to have as much variety in what you're composing as possible.
Hope this helps!