Every class is different, and you can only be in the class you're in, not in the one yesterday, or the one tomorrow. I've got a lot of issues with balance and leg strength. So it takes a long time for me to do some postures with any proficiency; other postures are fine however. Also, sometimes I can deal with the heat just fine, sometimes the yoga truck comes by and just runs me over, but then the next day I'm fine and suddenly I'm in the yoga taxi.

I figure a big part of the practice is just moving with it and not getting too much attached to self judgement about good classes and bad classes. I try to enjoy the good experience but not hang on to it, and I try to just move through the bad experiences and not hang on to it.

I'm pretty structured in my days and the practice, because I'm doing a challenge, so I go about hydrating and eating much the same way every day, and yet, every day, I can notice huge differences from one class to the next.

Just practice and you'll get more flexible. I did, amazingly in my first 30 days of classes. I never expected that! But my body was (and is) wiser than I am. The way it works is that the practice has to undo all the damage you've inflicted on your body by compensating for injuries your body sustained. It has to wind back all that conditioning and that just takes time. Be patient, keep practicing. It's an incredibly powerful practice. Good luck!

That's what I love about Bikram - that it's a huge adventure! Your body goes on so many ups and downs. Over the course of my practice (11 months) I've gone through so many different periods - strong standing series/awful floor series, awful standing series/strong floor series, awful classes, amazing classes. I hit a plateau around 6 months where nothing was improving, then around 8 months my practice improved dramatically!
I love that no two classes are the same for me and I love celebrating little improvements.

Can I just say, as a teacher, I love to read what you guys are saying to each other? You're exactly right. The only "bad" class is the one you don't go to! Otherwise they are just different (and yet also the same!).