I have no exp with this particular benzo, sorry. But I suppose it's possible that it just isn't very soluble in PG. perhaps a strong proof ethanol might work better but I have no idea. looking up it's information on places like pubchem they only list what I've typed below for solubility, doesn't mean it's not soluble in other stuff like PG, ethanol, etc but it seems from your exp that perhaps it's not so soluble in PG, assuming your methyl clonazepam is actually methyl clonazepam of course.
Here is only solubility info I could find:
DMF: 25 mg/ml
DMSO: 25 mg/ml
DMSO : PBS (pH 7.2) (1:1): 0.5 mg/ml
I don't know anything about DMF but I have used DMSO to dissolve Bromazolam a few times. Bromazolam was annoyingly stubborn about dissolving in PG especially when trying above 5mg/ml, but it was easier to dissolve in dmso. Like way easier. PG took all kinds of heat, tons of shaking, and plenty of time to fully dissolve whereas with dmso it just took a little bit of heat and a few quick shakes and it was all dissolved (usually I did it at 2mg/ml, don't know how high it's soluble to)
So you might have better luck using dmso. Can be purchased relatively easily online even Amazon offers it (should be ok to mention that since this an HR related item). DMSO can be taken orally in small amounts and doesn't taste too bad either in my opinion. Looks similar to PG, just a clear and kinda thick fluid.
As far as I know PG and DMSO mix together without issue. Meaning you should be able to add some dmso to your PG+methyl-clonazepam solution in order to get the benzo dissolved. If you do try this though don't do it with the whole solution you already made in case this too doesn't work out. Measure out a small sample, add a measured amount of dmso and see what happens. Will likely still require a warm water bath and some shaking but I bet the methyl clonazepam will finally dissolve. How much dmso at a minimum you'll need is hard to say, that's why I suggest starting with a small sample of the PG/methyl-clonazepam solution so that you can systematically add dmso until you find what works.
Good luck.