Unless you are working a 48 or 60 hour contract, then yes, your agency is doing well by you. There is an article on overtime negotiation on PanTravelers you might want to read that I couldn't possibly cover in forum posts.
Here are a few points though. There is no reason why you can't negotiate overtime separately from your base rate. At a minimum, you want it to be at least time and a half your hourly rate plus your per diem if you think that it is likely that you will be working overtime.
Not all contracts pay time and a half on the bill rate. A major industry player is ensuring that the bill rate plus $10 is not uncommon for overtime. I've worked contracts with a straight bill rate for all hours but those are unusual. The reason I mentioned 48 or 60 hour contracts is that those contracts have often have a flat rate until those hours have been met. There are also a number of hospitals in California that pay the first 60 hours flat (but double time if over 12 hours are worked per day).
No matter how the agency is being paid, this is how you should think about overtime. If the agency has contracted you for 36/40 hours, their compensation to you is based on that number of worked hours. If you work those hours, all your costs - hourly, per diem, housing, and travel - are covered, along with what they consider to be a fair gross profit margin to cover their costs. All the hours worked over that contracted amount are extra profit, pure and simple. They have no extra costs because you work extra hours other than the extra overtime pay.
Here is an example to show how the agency benefits at even a straight bill rate of say $60/hr, paying you $30/hr. If you work overtime at $45/hr, they are making a pure profit of $15/hr. All their other costs have already been covered (well, less some payroll tax). If you have a base rate of say $20/hr and OT of $30/hr, you can see that they are making out like bandits. Even with a straight bill rate.
So you do want to push for max OT pay. But you also have to pick your battles. If working a lot of hours is what you really want to do, you might want to consider an agency such as Fastaff. If you know that there is not overtime on an assignment, you might want to negotiate for more in another part of your compensation instead of fighting for something that won't benefit you. And if you don't want to work OT, why are you doing it? That is the point I would raise in negotiating for higher OT pay: Do you, the agency, want to give me an incentive for making you more money? Otherwise, forget about it, I'm not working overtime.
If you want to mention the name of the agency and hospital, I might be able to tell you specifics about that situation.