There is a bacterial alpha amylase paper that says the pH for solubility is optimum at 10. And the salt concentration affects solubility. In sodium sulphate, for example, the solubility is poor. I would pH your solution to a higher pH. The preparations, especially prepared from the gut, are at low pH according to the literature. So I would try to resuspend the protein, and pH your solution to make sure the pH is high enough. I just googled solubility and amylase.