Dear Gideontan;
Several cases are thinkable:
Your Nickel maybe is not what you think, or you may use a too strong HCl, or you are not moving the sheets in the solution too.
In the both later cases something similar to “passivation” will come into the place.
I think you have to check for all cases, because the solubility is about 255g / 100ml water (25°C).
Good Luck!
ARGOS++