you probably need a fresh perspective, especially if you're life is consumed with one particular piece. if you can, try taking a break from that one piece and work on something totally different for a day (try to get it out of your head completely). then come back to it and see if you can make any head way.
if that's not an option then get another engineer, who's opinion you respect, to come in and just listen to it with you. the producer doesn't have to be there when you do it. the opinion of someone who's not so invested in the project may help you find the answer you're looking for.
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When you can't get away from the project for a bit as Shaun suggests (which is THE best suggestion) the alternate is to move on... i.e. just leave it FOR NOW and move on to the next part of the mix. If the Producer is there just say "We're getting stale on this, let's move on and come back to this with fresh ears in a bit", then continue mixing the rest. Come back after you've gained a little space. Not as good as stepping away for a period of time, but a good second.
A lot of times I find myself getting too caught up with the tiniest of details until I get a colleague in and they just dont find that 'click on that word exactly two seconds before the big explosion' annoying. A second perspective always helps!