Did you buy a colony style type trap?
Colony trap

Or a body trap like a Conibear 110?

Conibear 110

You can catch 1-6, maybe 7 muskrats in a colony type trap. Only one at a time in a Conibear.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).