There might be a few factors for you to consider. Do you have good quality pedals? Are they true bypass or buffered? Are the jumpers between each pedal in good shape? Are the guitar cables in good shape? ow long are the cables? Etc. Lots of factors.

A compressor will help your sustain, but at the cost of dynamics. What the compressor does is makes the quiet sounds louder, and makes the loud sounds quieter. It does this by "compressing the signal".

Visualize sound as a waveform with peaks and valleys. The compressor will put a ceiling on those peaks so that they don't exceed a certain volume based on what you have the compressor knobs set at.

The quiet sounds are boosted and this is where you get the improved sustain. As your note fades, but remains "in range" of your settings, it gets boosted. Kind of like if you struck a note to let it ring out and fade, and while it was fading, you slowly increased the volume on your amp.

And, there go your dynamics! :(

At least that is how I understand it. Hope it helped.