According to your motherboard manufacturer, the only supported Mushkin memory is part no. 995659 and they are 2GB modules. I don't know whether that infers no other modules will work or not. Even Crucial (which normally has a number of options), can only offer 4, but they are 4GB and 8GB modules. It therefore looks like, there is limited support by the motherboard manufacturer in this instance.
As with many dual channel systems, it is important where you install the modules and as they don't match each other, if you haven't already done so, try installing one matching pair in slots 1 and 2 and the other pair in 3 and 4. Note, the slots do not run consecutively. Facing the motherboard with the external connections to your left, from left to right the slots are: 4, 2, 3, 1. In other words, install a module, miss a slot and then install the matching one.
I don't know if that will help, but it's worth a try.