It's often useful for me to examine when and where I'm having the most fun, while struggling with a work of fiction. If I find myself "playing favorites", i.e., preferring to spend time with a secondary character, it's time for a reevaluation! Nothing is worse than being bored with your main character. Duh...your reader will be bored, too...
I've been known to ditch my main character and hang out with that livelier, funnier, more mysterious and interesting secondary character, thus changing my story almost completely.
More often, I merge the two personalities, discovering new plot insights as well. This is a good example of how my "block" reveals valuable information, when it is actually a positive aspect of my writing process.