Hello "IHC TISSUES" !!

Based on your explanation, you have 2 options:

1) Try using ice cold HCl. And if the powder still did not dissolve you could keep it longer on ice or leave it over night at 4 degrees until all pepsin powder is dissolved.

2) Alternately, you could use an intermediary solvent (1 ml or less ice cold double distilled water) and dissolve the pepsin powder initially in it, then add this amount to your cold HCl. (If you are preparing 1 L of pepsin-HCl, 1 ml DDH2O would not make much difference, and you should check the mixture's PH after that)

Best regards,

Ahmed