Asking for a bigger commission is one of the easiest things to do, yet most people are reluctant to do so. All the affiliate network, or affiliate manager can do is say no. So there's no harm in asking.
I would suggest that you prove yourself first though. Find some offers within their network and start selling them. Once you find one that converts, and it proves to be profitable for you, it's profitable for the affiliate network too. The more money you make, the more commission they make.
Call up your affiliate manager on the phone and ask them to take a look at your account. Show them the numbers. Then ask them for a slightly bigger payout on the offer. If there is any wiggle room at all, the affiliate manager will usually say yes, rather than lose your promotional efforts on that offer.
If the offer is exclusive to that affiliate network, it may be a little more difficult to "get a raise" as it were. But if you search around the other affiliate networks and find the same offer, or a similar offer paying more, you could mention switching to that offer. In this case the affiliate manager almost always says yes.
At the very least, there should be some sort of negotiation. The affiliate manager might set a certain target, saying hit this number and we'll give you a higher commission. Jot down the date and time, and then hold them to that commitment in a future conversation.
Speaking of conversations, have you met the affiliate manager in person? If not, consider attending some of the trade shows and events that the affiliate manager also attends. Just ask them which events their affiliate network will have a booth at in the next six months. Then show up and start a relationship.
People are far more likely to do business with, and continue to partner with, people they know and have met in person. If I had to put a number on it, I'd say roughly 20% more income, simply by showing up, meeting people and making friends.
Make all decisions based on the person that you'd like to become