What does it mean if your banners have high CTR but not so high CVR?
Is it that the user thinks what the banner tells them and what they find when they click through aren't similar enough?
Or is it better angle creation needed?
Better copy overall?
Or is it just down to luck?
Or something else?
At the moment I'm running 2 campaigns with approx 1.5% CTR on banners, but the CVR is not as high, and it makes me wonder why I'm getting so many click throughs but not then more conversions.
I think you didn't mention which vertical you're running at, but it actually doesn't matter that much. While high CTR is important (cheap clicks), it's not everything. Just take the following example:
Traffic source: whatever you name it, no matter mobile or web
Offer: car insurance
Banners / Creatives: $100 stripclub vouchers for free!
CTR = BOOM
CR = none
While it might sound a bit lame as an example, it's pretty much your problem. You seem not to convert the clicks to the lander/angle to the offer in one flow. Getting clicks is one thing, making them somehow targeted is the golden nugget to combine with high CTR 
First thing: sometimes you'll find a banner with a wicked high CTR, but a terrible CVR, and no matter what you do the bloody needle won't move on the CVR. They're super-annoying, but just make a note somewhere of that banner in case you suddenly get an idea to monetise those clicks, then deactivate the little bastard.
If your CTR is, in general, high, but your CVR is low, that usually means that you've got a problem with the "flow" of the campaign. Either you're sending people to the offer without enough incentive to click, or there's something in the banner -> LP -> offer (or banner -> offer transition if you're direct-linking) that's putting them off.
One of the most common things that puts people off is if the "appeal" changes - as fjk87 says, if you're advertising free stripclub vouchers then sending them to an offer for car insurance, you won't get many signups. Have a careful look at your funnel and see if you can spot any places where people could think that what they're being offered has changed, where they might think they're being scammed, or similar.
If you're getting very high CTRs off one placement, in mobile or PPV for example, that might also mean you're getting accidental or bot clicks. If they ain't converting, block that placement.
If you haven't read this, you may find it useful - on a related topic, but has some very good suggestions. http://stmforum.com/forum/showthread...y-Don-t-Matter