Confessions of a Cat Breeder

I have a problem with my male and female siamese cats, Mindy and Tong Po...I don't really know where to start but Mindy has been coming in heat for the past 6 or 7 months and Tong Po is having trouble breeding her...to make a long story short, he stands on her back and thats it...he knows something is going on but he can't quite get to where he needs to be, if you know what I mean...I don't know if Mindy is not keeping her hips up high enough or what, but he just can't get where he needs to be...and then sometimes when he is straddling her she rolls over onto her side...is there anything that I need to do as far as seeking the help of a vet?...or do I let them just figure it out for themselves...she's never been bred before and he has never bred either...someone please give me some advice on what needs to be done...thanks