I'm told Twitter is evolving into a channel that's primarily for customer service, not so much for sales. Trying to force the channel to provide you with tons of good leads may be a waste of time.
Google+ at the moment appears to be mostly occupied by tech-savvy geeks. Is that your target audience?
Different channels appeal to different audiences. Marketing on Pinterest for an industrial equipment supplier might not be the best use of someone's time, because generally speaking industrial equipment buyers aren't hanging out on Pinterest.
If your audience isn't hanging out (yet) on G+, it's a waste of time trying to force the people who are there (now) to visit your site and buy from you. They aren't your target customers.
Now, from what I hear it's possible that G+ will be growing and attracting a different crowd in the future. So it would be good to keep a presence there, post occasionally. But until your target demographic shows up there in big numbers, it might not be worthwhile trying to force more traffic/sales out of that channel just yet.
In other words, just because you get more traffic from one channel than another, this doesn't necessarily mean there's anything wrong or anything that you should change (unless you're spending a lot of effort trying to force additional traffic out of the lower-performing channel with no success, in which case you should consider stopping that unproductive use of your energy and time).
As to why you're not getting conversions, there could be any number of reasons. Some of the most common:
1. You're not making the case effectively. When the visitor lands on your page, you're talking about features, or focusing on benefits that don't mean that much to them. Find out what your prospects consider to be their biggest pain points, and speak to how your company can ease their burden.
2. Ineffective (or missing) calls to action. You can't assume that people will naturally know you want them to sign up for your newsletter, buy your product, request a quote, download your software or whatever it is you want them to do. You have to tell them. Sometimes you need to tell them several times on the same page. Don't leave it to them to figure it out on their own.
3. Asking too soon. Especially if the product or service you're selling is expensive or requires a commitment on their part, you need to give them some intermediate steps to ease them in to the purchase. When someone first arrives at your site from a social media link, asking them to spend a lot of money or commit to a long-term contract is a bit like asking someone to marry you on your first date. Sure, it could
work on the odd rare occasion, but more often than not it will just get you the cold shoulder and an early end to the date.
So maybe instead of asking them to buy right away, you offer them a white paper. Or try to get them to sign up for a free newsletter. Or you just give them some good information and try to get them to "like" you or follow you or whatever, so you can continue to send them more good info. Eventually, after you've built a relationship you can try to move them forward toward a purchase.
There could, of course, be many other causes. That's one reason why conversion optimization consultants can charge so much money to diagnose and fix those kinds of problems.