Shedding Issues in Large Pythons?

I take care of a large number of big pythons. Most shed fine and clean, except the four that range between 15-20 ft. These snakes have to be assisted after each shed. There should be no husbandry issues-they have a nice gradient of temperatures to choose from, ample size ponds to fully submerge and swim around in, not a lot of physical contact.
Has anyone else experienced poor sheds from larger snakes? Does anyone have any theories as to why, or solutions that solve the problem?

Thank you for your insight!

Whats your humidity levels at?

I've never heard of bad sheds being size related. Thinking it must be a husbandry or health issue.

I have friends whom are zoo keepers looking after reptiles and I know they rarely have single sheds of snakes that size but they always are able to complete the shed un aided. I might suggest that perhaps a faulty hygrometer. Digital units are easily altered to read wrongly. Damage to cords, battery faults (low poor contact) dirt on the probe, dust, urine, any type of salt or conductive or resistive material on the probe even minute amounts will cause hight or low readings. (they measure the electrical resistance of the air...) I use high quality analogue ones as they are much more robust. It might be worth the effort to check the hygrometer for accuracy. Normally I'd suggest a salt test but this test damages digital units. You can buy a calibration kit from a cigar store to use. It might be cheaper to just replace the hygrometer with a certified new one.