The nape of the neck on a full cap can be a tricky area to keep securely attached with tape or adhesive. This is mostly due to the movement/turning of your head which causes your neck area to stretch. If you are not already using a strong tape for extended wear I would recommend switching to one. (tape is easier to change). If you are wearing your system for days to weeks at a time you can change the tape just at the nape area every week or more frequently depending on how long you keep it on.
Also make sure the length of your system is correct and doesn't go down too low on your neck. Your system should follow where your natural growning hair line would be. If it is lower that could also be the cause.
I hope this is helpful.
Have a great day!
Lisa | Hair Replacement Consultant | firstname.lastname@example.org