I'm not sure if that last bit is kosher, but I'm not positive so I'll leave it for other mods to decide...there's only ONE place in that country slinging GBL, and it's a fairly popular source, too...
In response to your question, though, since GBL depends on lactase, an an enzyme in your stomach, which some people are deficient in (lactose intolerant individuals) it makes perfect sense that some people would be weak responders. (are you lactose intolerant?)
Try converting the GBL into GHB (look it up, my little sister could do it

)
You should have much more consistent dosing with GHB.