I just started playing guitar about a month ago so I recently got a capo but the problem is that it doesn't hold the low E string down all the way so it doens't make a crisp sound..
are you putting the capo in between 2 frets or on top of one? you want it in the gap, so to say.

this seems like the only thing i could think of

otherwise, maybe you just got a really cheap capo or your guitar's neck isn't perfectly straight
My Guitars:
Gibson Les Paul Studio
Epiphone AJ
Ibanez Strat Copy

Orange Tiny Terror Head
Old beaten up Peavey cab
Marshall MG30DFX