I have had tons of trouble with hollow body holds and was basically brute forcing them as they never got easier.
I realized that I have not been using my deep abdominal muscles to aid in contraction of the hollow. my stomach would come up in the middle leaving a little "mound" rather than a flat core. Today I worked on controlling this through breathing and actively flattening and was able to complete 60 seconds much much easier. It also turns out that I was slightly favoring one side which also limited activation.
Just thought you guys might benefit from this discovery if you are like me and don't have the correct activation.