ya like others said always keep in contact with the artist. To make sure you know what is up. Now if its a bigger name guy. Sure there could be variety of reasons be it they have other commissions they are also working on. To if they are currently employed by x publisher they have there paid gigs they have to take care of first. To if they have some personal things going on. Like a death in the family, just having a kid, got sick, or some times there is some folks that just take awhile to get work done.