I do have one of the December purchased silkies with a persistent crop issue, it is still really slow, I checked her again this morning and it is much too full and squishy. She's on Day 6 if the acodofied copper Sulfate. I think I'll change her to probiotics and plain water tonight. I'm at a loss as what to try next.
I've found that if the Copper doesn't fix the crop issue, something else is going on to cause a yeasty crop. Mareks could definitely be weakening her system. If you don't see any changes by day 8 or 9, Id stop with the copper. Have you tried Corid? Coccidiosis can slow crops down and cause serious weight loss.