Everything seems to be going just fine: You've got freshly ground coffee brewing in your French press, your timer ticking the seconds away until that precious four-minute mark. You're mere moments away from enjoying the first cup of the morning when—uh oh—the plunger won't depress.
In your caffeine-deprived state, maybe you succumb to the obvious option and push harder. And then harder. It won't budge! One more push and then...
OH NO—SHATTERED FRENCH PRESS, HOT COFFEE EVERYWHERE!!! AAARGH!!!
What happened, and how can you avoid coffee winding up everywhere in your kitchen except your favorite mug?
This actually happened to me two weeks ago, five minutes before the beginning of an 8 a.m. conference call for which I always desperately need a cup of coffee. Yes, even though I knew better, I kept applying more and more pressure to the lodged plunger, eventually causing the beaker to shatter under the strain.
It was a rookie mistake, but it doesn't have to happen to you.
The reason the plunger stalled is because of a buildup of what we call "fines" among the coffee grounds, which get clogged in the press's metal-mesh screen and jam it up, which prevents it from moving easily through the brewing liquid. The term Fines describes a kind of pulverized coffee dust that most grinders will create, even when on a relatively coarse setting.
Lower-quality grinders—say, the whirly-blade type—and those with dull burrs will create more fines. In the former case, the blades whip the beans around somewhat at random, creating uneven particle size within the same batch. (While some beans get the blade treatment over and over again until they are reduced to powder, others get roughly and coarsely chopped.)
In the latter instance, the fines are created because the dull edges of the burrs need to be brought closer and closer together over time to create as fine a grind as they did when they were fresh; at a certain point, they will be so close as to simply decimate the beans, as in a mortar and pestle, rather than more acutely chopping and grinding them.
"Whatever you do, don't force it!"
All that being said, what should you do if you find yourself with a press pot you can't press? Whatever you do, don't force it! Simply apply a little pressure to the plunger, then lift it back up about a quarter of an inch, to dislodge the finicky fines. You'll usually find that backing up on the plunger once or twice will loosen the coffee glue that's jamming up the works, allowing you to depress the thing at last with ease.
And that's better than a kitchen floor full of hot coffee and soppy grounds, right?