My cat has play aggression, but refuses to play?!

I have two cats, one a little over a year old and one a little under.  I got them a couple of months apart, and did the whole gradual introduction, site swapping, etc, and within a couple of weeks they were playing together (mostly) without any complaints, napping together, grooming each other, etc.

Everything has been fine until recently.  Suddenly, the slightly older one will NOT play if the younger one is playing.  She won't go after the same toy or anything, just watches.  It's like she's thrown off by the younger cat's energy -- but when she's alone, she is just as energetic and enthusiastic as the younger cat.  Unfortunately, my kitten refuses to entertain herself while I play with the "big sister," and if I shut her in a room, she cries so much that it distracts our older cat and she won't play (plus, I feel horrible).

I'm pretty sure this has led to our older cat using the kitten as basically a toy.  More than once in the last few days, she has cornered her (this is not normal for them), and I hear crying, squeaking, and growling more often than I used to, though no injuries yet.  They still nap together and groom each other on a pretty regular basis.  Should I not be worried and just let them settle things on their own?  Is there a way I can encourage them to play together?  Or at least in the same room?  The only way I can get them to have equal play time right now is if my husband takes one cat upstairs to play and I play with another downstairs... even with two of us and two toys, the younger kitten will go nuts and take over our older cat's toy.   And of course, as soon as playtime is over, "big sister" is stalking our kitten and chasing her down... :/

Any suggestions?

Once cats have been introduced and are doing well with each other, it's best not to separate them again. They can have their good days and bad days (most relationships between humans have those as well) but it's important that they keep sensing each other so as not to become estranged. 

It's possible that the older cat is settling down and their temperaments are not as matched as they used to be. It's also possible that they are re-organizing the hierarchy of their small two-cat clan. Either way, it's out of your control and all you can do is allow them to conduct their social changes in a supportive environment. 

I have two suggestions for you at this point - 

1. Watch your own reactions when they are together, especially when they're interacting in a hostile way. It's crucial that you remain calm. It's best not to intervene and let them sort out their own agendas. Any negative response from you could potentially make the situation even more stressful, possibly to the point of them associating each other with negative sensations. We do not want that to happen.

2. Try to create positive associations when they are around each other. Give treats but make sure the cats do not have to compete over them. It's best to start by throwing the treats to each cat so they can enjoy them at a distance. Play interactively with the younger cat when the older cat is around (again, make sure one does not run into the other during the playtime sessions).