Gabapentin capsuls, now on pills, not working

I was on 2-300 mg of gabapentin capsuls at a time, they worked fine, even a little "high" going on. This was every time I took it, for months and months. Then Dr switched me to 1-600 mg pill form and now nothing, even if I take two at a time, nothing! what is up with that??? are the capsuls better absorbed, or something?? I don't understand what is going on!!