Good advice all around. Here in the US, temp wages tends to be about 2/3 - 3/4 of the amount paid to an agency. So, if you get (for sake of argument) $40/hr, the firm would be paying $53 - 60/hr. If OT is paid at a rate of time and a half, then every hour of OT is costing the firm $75 - $90 (given the figures I'm using)! No wonder you aren't being given OT.
Law firms are inherently busy places and schedules can often be chaotic. As a PA, I take it you are more of what's called a legal secretary here, rather than a paralegal (I got my info from here:
http://www.tpas.company.org.uk/ if I'm mistaken, please let me know). If that's the case, then a lot of what you do is probably rush jobs and rescheduling of appearances and depositions. If these tasks cannot be performed, the clients suffer - while you don't want to specifically say that to your boss, that's the gist of what can happen if the work is not performed. If a complaint isn't answered on time, clients can lose some of their rights in a lawsuit. So it's a big deal if work is not accomplished on time.
Hence, if you are able to plead your case with her, either in a personal meeting or at the review itself, you may want to approach it that way, as an issue of client service, rather than "this is how you did it in the past" or "all the other PAs get OT". By the way, it's possible that not all the other PAs get overtime - this wouldn't be the first time that a fellow employee tried to screw with someone's head. So be aware, the info you're getting from your peers may not be accurate.