[XPM] Primecoin mining help needed

[caz money]

I have 8 rigs that I use for litecoin mining.

4 rigs with:
4 x 7950 gigabyte
sempron
1050w sparkle psu

4 rigs with
3 x 7970 gigabyte
core i7 3770k
1200w corsair

I have installed the XPM high performance mining software on all the rigs, but I  have only found 4 blocks in 4 days. The weird thing is that they are only found on 1 rig...The rest of the rigs have been mining XPM as well, but have not hit pay dirt. Is this just coincidence or is it possible for me to have done something wrong.