I've developed my own STB extraction technique - all fairly obvious stuff with a few twists - and I nearly always end up with about 1.25% yield of fluffy white crystals. I've just gone to a batch of MHRB that I'd previously got a good yield from and done an extraction only to find that the overall yield was down to about 0.75%. The MHRB has been kept in a sealed plastic bag at a temperature of around 20 degrees centigrade for the last 8 or 9 months.

Would anyone expect it to lose potency in this time? I can't see that my extraction technique was any different to a dozen times before.

i am currently doing an extraction with MHRB that was in my closet for about 2 years.
currently my yield is at about 0,75% on the 4th pull.
but this is also my very first extraction and i had to wash and re-ex some because i was not working clean enough

edit: i am doing Dream Weaver's tek STB with heptane

What temp are you pulling with?
Heptane requires higher temp to pull than Naptha etc, around ~40 deg C..