It's not uncommon here either but there are things you can do to improve it in many instances.
One of the keys is managing player "expectations". If you give a player incorrect information or raise their expectations above what they actually get/see on the other side then there is a less chance of them converting.
A small example: a) you tell a player they get a $500 bonus. If it turns out it needs multiple deposits your chances of a conversion are less than if you said the bonus was $100 and they get to the site and find not only is it $100 but they get a bonus on deposits 2 and 3 aswell.
Another common mistake I have made is to advertise the wrong casinos in the wrong places. For example, if I get hits from Google on "Lord Of The Rings" and I run promos for Playtech casinos on the landing page (who dont have that game), players will often sign up to see if it's there, find it's not and go somewhere else.
Of course there are plenty of factors outside your control too (deposit methods etc) but you can increase conversions to a degree by optimisation.
I've come across stats that suggest it's 5-10 times easier & cheaper to retain a customer than to gain a new one so working on converting the traffic you already have is more financially beneficial than getting more traffic (obviously a balance is needed).
GL though. It's still an excellent business to be in compared to other affiliate niches if you get it right
Cheers
Simmo!