Have you adjusted the bias trimpot as described in the instructions?
Quote:
When you have finished building your Mimosa Jr. kit, you will need to adjust the bias trimpot before the pedal will actually compress. You want to adjust the trimpot so that you get approximately 1.5-1.7V on the source of Q2. This would be the āSā eyelet to the left of the trimpot. Or you can just use your ears if you know what compression sounds like. Be sure to turn the BLEND knob full turn clockwise so that you are hearing 100% wet, compressed signal and no dry signal. You will have a very hard time trying to adjust the compressor bias by ear when you are listening to the dry, uncompressed signal
I'm not sure an unbiased pedal would sound the way you're describing, but it's the first thing that comes to mind.