I'm having a little trouble following your description of the problem, are you saying the compressed section of the note is too loud or too quiet?
The compressed section is too quiet. The attack is loud and punchy and the compression kicks in directly afterwards (like it's supposed to do) but it compresses too much and brings the level too far down even on the lowest "sustain" setting.
Do you have the pots fitted the right way around - A100K for volume and B500K for sustain. It is possible during the build to confuse the front and back views of the PCB and of course the component view will show the 500K on the left and the track side view will show it on the right. And you can't see this once the effect is mounted in the case, you have to remove it from the case to read the values printed on the front of the pots. However if you have a multimeter and know how to measure resistance you can measure across the ends of the pot in the Volume position while it is in circuit to see if it is close to 100K.
You know, I made a point to double, nay, triple check which was which but it's such an easy thing to overlook that I might just have to pull it back out and make sure which is which. That'll also give me a good reason to redo the semi-shoddy but functioning footswitch wiring.
Maximum gain is limited by the ratio of R4 (1000K or 1M) and R5 (10K) to 100. The sustain pot is in series with R5 and if it is a 500K, then turning it up to maximum resistance should give a gain of about 2. If you have the 100K in that position then even at max resistance you still have a gain of about 10.
What are those numbers "gain of about 2/10" describing? Is that referring to decibels of gain, decibels of gain reduction, or something else completely?