Wait..what do you mean, they explode? I have a hen sitting on eggs for what seems like forever and no chicks...but shells and the hen looks like crap.
My hen Sat a small clutch of eggs, she was really stubborn about the last two, I had her in a broody cage, Idk if she stepped on it, or moved it wrong or what, but it popped, and got all over her and the nest and smelled terrible!
Has she been sitting longer than 25 days? If she has and they haven't hatched, then she's sitting on little stinky times bombs!
If it's only been 20 days, then it should be any time now...when she gets off the neat next you might want to check for pips, listen for peeps, maybe candle...try not to move them too much though...