I've had this problem and without going through the usual, "genetics" talk, I'd say just try something different than anything you've done so far.
Maybe try: giant sets, drop sets, rest pauses, supersets, or BFR (blood flow restriction or occlusion) training.
In case you don't know; in a nutshell:
Giant Sets: Begin at a relatively low weight and do ~15 reps. Then, move up in weight enough that you hit ~10 reps then up in weight and do ~6 then up again with ~3 reps. That's one set. Then take a 30-45 second rest and do it all over again. Those are the rep ranges that I like to do, feel free to change it up for what works for you.
Drop Sets: Start at a comfortable weight and rep it out until failure, then drop the weight ~5%-20% and rep that until failure (2-4 reps). You can continue to drop the weight until you are satisfied with your set.
Rest Pause: Begin at a comfortable and mediocre weight for you and rep until failure. Then, wait ~15 seconds (some say take 15 breaths) and do that same weight for as many reps as possible. I sometimes do a drop set into a rest pause.
Superset: The superset system employs two exercises performed in rapid succession. There are several different ways to superset: either supersetting between two like exercises for the same muscle group, or perfroming two exercises back to back that involve antagonist muscle groups (i.e. chest and back or quads and hams). Obviously, for calves, you will want to the former. I usually begin on one calf raise machine and then move to another. That would be one of the only ways to employ supersetting for calves.
Occlusion Training: is a hard concept to explain on a forum without going into extreme detail. You can google BFR or occlusion training if you would like. Again, in a nutshell, one will cut off veinous retrieval of blood (occlude) using a pressure cuff (I usually use resistance bands) without blocking off delivery of blood from the arteries. *Yes, this is a safe and scientifically proven and effective way of building muscle and increasing strength.
Also, if you have not yet, you can try angling your feet differently on the machine itself. This meaning: start with your regular calf raise and rep it out, then angle your feet outward (about 45 degrees) and rep that out. then angle your feet inward (about 45 degrees in from your original position) and do more reps. You can either do this as one big set, or break it up into three different sets.
I hope this helps! I know many people know what a lot of this stuff that I have explained is, though I feel it can help some people who are newer to lifting that are viewing this post, so I am sorry if I couldn't help with this post. Hopefully I have helped someone! If you have any other questions, would like more of an explanation, or would like some good links with info on these topics, feel free to message me! Happy Training!
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