My company offer a similar service with A/V and automation equipment, and I pay helpers to work on a casual basis too. I don't generally find a problem with the workers, but have had one or two just not want to be there. I think that most of this is because of the out of hours nature of the work, they agree to work, then find out that there's something better that they'd rather be doing. As this is an extra income to a full time job they could just not do it and go out with their mates/family instead which may be on their mind? Never really found a solution to this, except to try to give as much notice and planning as possible to make sure everyone is as ready mentally as possible.
If you ever need help with any A/V, automation or lighting stuff then let me know?