My general opinion is that G2Crowd is a BUY stage play, for leads that are searching for terms like ‘x vs. y’ or ‘x reviews’ with ‘x’ being your company of course. I’d take a look, if possible, at what search terms brought visitors to your G2Crowd page (pretty sure G2Crowd is building out their analytics to be pretty robust). This will give you an idea. So, if you’re looking to accelerate a lead who is beyond the discovery/education stage to purchase, then it is a good play.
If your focus is lead generation, I don’t think that G2Crowd is that great. As Miguel mentioned above, Capterra does quite well too. And my experience has also been do Capterra first, then migrate to G2Crowd as a land and expand play. Capterra is also cheaper so you can test the waters.
Lastly, what CTAs are you putting on your premium profile? That might matter. If it’s not relevant to the lead’s stage, then that might be why you are not getting conversions you’re looking for. There is also space to put specific content.