timers 2 and 3 on MSP430F6736

I've programmed up all 4 timers, A0/1/2/3 for simple count up with identical parameters. (all running off SMCLK which is set to 4MHz)

I use timerA0 for an interrupt set to 1kHz , which is running fine.  I'm running some PWM off A1, also fine.

But timers A2 and A3 aren't counting.  TAxR is zero and stays there.  This is pretty trivial stuff, but is there something different about these two timers?

If I change the clock source from SMCLK to other things I can sometimes get a single count in TAxR so it appears to be a clock problem.  It there some fancy mapping in this part I'm unaware of?

Gary