Assuming theres nothing defective with any of the components...
Make sure the 2 10's are in phase with each other.
Make sure you have wired the voice coils for an optimal impedence load for
that amp.
Make sure the enclosure has adequate air space, and no air leaks. (very
important)
make sure the subs are receiving all freq's and you dont have a high pass
filter engaged on the amp or head unit.
make sure the rca cables are delivering the signal to both amp channels.
make sure the amp is receiving adequate voltage from the electrical
system.
Double check all speaker, power and ground wires to ensure solid
conductivity.

In addition, try repositioning/reorienting the box.