To paraphrase Dan John, efficiency is the enemy of fat loss.
When people start running for fat loss, they find that as time goes on, they're having to run farther and farther in order to get a training effect. The problem is the SAID principle (your body adapts, and becomes more efficient at what you ask of it; more efficiency = more conservation of energy = less calories burned).
The easiest way to combat this (and thus carry on burning calories and losing weight)? Do something you're not very efficient at!
Without knowing your exact routine, it's difficult to give exact prescriptions, but to answer your general question, should you increase the repetitions, I would say... maybe.
Increasing the reps will place a greater demand on your body to adapt (burning more calories), but as with the person running for fat loss, if all you're doing is increasing the reps, you'll have to keep increasing them, and there will be a point of diminishing returns (if you haven't already hit it), and you'll also run into a time issue where your gym sessions start taking up a lot of time.
One suggestion would be to increase the weight and decrease the repetitions per exercise, and although that is a good option, I would suggest instead changing the exercises you're doing as part of your training sessions, maybe completely changing up your routine.
For example, if you're doing a body part split routine (which it looks like you are), you could change to doing:
- Monday: Full body training (do a hinge, a squat, a push and a pull movement)
- Tuesday: 20 - 30 minutes cardio (make it something you don't normally do. If you're a runner, then row, if you're a rower, then use the bike.
- Wednesday: Full body training (do a hinge, a squat, a push and a pull movement)
- Thursday: 20 - 30 minutes cardio (as Tuesday, something you don't normally do)
- Friday: As Monday and Wednesday
- Saturday & Sunday: Go for a walk, get outside, enjoy the sun (if you don't live in the UK, we never get sun), go spend time in nature, paddle in the sea, whatever you fancy that isn't sitting on the sofa watching Netflix non-stop.
You've mentioned you do crunches on Monday and Friday, so instead of those, if you want, throw in a quick core routine a few days a week (I'd say Tuesday, Thursday and Saturday if you don't want to do it after your full body training days) and look at doing more support or bracing movements, anti-extension, anti-flexion and anti-rotation.
Speaking personally, my biggest change in body composition came when I switch from a 3 day a week powerlifting based routine to a 4 day a week calisthenic (bodyweight) routine. It took me a few weeks to settle into the routine (the warm up alone was leaving me a sweaty mess) but once I did, I really enjoyed the change of pace and different way of training.
