I had this issue as well. Sometimes I'm given the option to cast spells and sometimes I am not, and I can't figure out a rhyme or reason to why - I always have mana open to be able to cast the intended spell. When I get the "resolve" button screen, I couldn't see how to cast the spells exiled with Voltstrider even when I try to click on them instead of clicking the resolve button.
I had this issue as well. Sometimes I'm given the option to cast spells and sometimes I am not, and I can't figure out a rhyme or reason to why - I always have mana open to be able to cast the intended spell. When I get the "resolve" button screen, I couldn't see how to cast the spells exiled with Voltstrider even when I try to click on them instead of clicking the resolve button.