To go from 20 to 100 you need to do more reps. Simple. If you're only doing 50 in a whole workout and never increasing you won't ever get to 100 in 1 go. Should do 100 in the entire workout for 3 days a week then add 50 every week untill you're doing 300 in one session then you will find 100 pretty easy. That's how I did it.