Employee with another job that conflicts with our workplace

Hello,
We have a PT employee that we like and appreciate, but they work a FT job with another employer.  They haven’t worked a shift in roughly one month for us.  
Is it appropriate to ask them to commit to a minimum shift requirement for us?  It is difficult to ensure they are involved in compliance training sessions, etc, in this arrangement.  As I mentioned, we like this employee and would like to keep them but we are caught in a space where they can’t give us any assurances as to when they can work for us, and we would like to consider asking for shift commitments.  Can we do this?  What kind of options are we legally allowed to consider?
Thanks!