What is good storage for Metol (and Hydrochinone)?
It's actually not all that critical. It just needs to be kept dry and preferably dark, that's all.
it's hard to gauge a problem from a post like this
Yes, a bit. A few things do come to mind.
Try to do A/B testing with a known-good developer and film you know is in good shape and well-exposed so you can exclude with all certainty a film-related problem. What film have you been using, and is it (reasonably) in date?
With the self-made d76, verify you're using the right amount of carbonate. If you use e.g. decahydrate in the quantity the formula gives for monohydrate, the resulting developer will be too weak.
As to your metol, if it's white, off-white or light tan, it's probably fine. Dark brown or black is not good. Metol keeps pretty well and doesn't go bad easily. In case of persisting doubts, acquire a second (test) batch of metol from another source. Even a tiny quantity like 5g of known-good metol begged off a photographer friend will allow you to do a comparison test and exclude this factor.
Verify your scale; ensure it's calibrated and that it's not used at the very extremes of its scale. E.g. using a scale that can weigh up to 1kg / 2lbs to weigh a 0.5g quantity is evidently a risky endeavor. I assume you use a jewelers scale or something similar for the small-quantity chemicals. The digital kind are cheap, pervasive and generally plenty accurate enough for our purpose. Some swear by triple beam Ohaus scales etc and that's fine, but not a necessity.
In other words, it's a matter of dotting some more i's in the hope you'll hit upon a previously missed parameter. Systematically go through the entire process of raw material to finished negative and take stock of all factors influencing the outcome, and theoretically and experimentally cross each and every one of them off the list.