Some memory just does not work well together. For example, G.Skill strongly suggests that you buy matched sets that have been tested together.
That said, I've run different sticks together many times. You must be running the same speed, the same voltage, and the same timings for ALL the timing settings.
If they won't run under those conditions then just move on and use them on different builds or sell a pair. 8GB is sufficient for pretty much anything other than memory intensive programs and lots of virtual machines.