I have gotten frustrated or bored with a WIP before, if its a frustration, I will put it aside and move on for awhile. Sometimes the lightbulb goes on while working on a new project, and I'll go back to it. If its a boredom issue, sometimes I hang it over the bookshelves for a couple weeks so I can see it all the time, and decide if it get worked on or put away in a couple weeks. Right now I think there are 7 projects going, one of them being test patterns for a designer. Plus a knit afghan, and three crochet projects. Of course, I am ADHD which helps... get bored? already another thing to spark my interest.
Then sometimes you have to bump other projects for necessity (its really cold here, so building quick bargello window quilts for drafty front livingroom windows... anything to warm them up!)