Thanks Wet Boots. Borrowing from your knowledge & my ingenuity, I put together a home made device to measure nozzle pressure using a pressure gauge, a foot of vinyl tubing, a garden hose repair piece & a tubing clamp.
With it, I found that the nozzle pressure (as you surmised) was only 9 PSI - nothing like the 28 PSI home water supply pressure measured at the outdoor spigot. This gadget also helped me to discover a reason why the pressure might be so low. There was one (seriously leaking) head that had sunk out of sight below the sod & was forgotten. Once I found & capped it off, the nozzle pressure went up to 21 PSI. Now the rotary nozzles rotate like they should.
Best regards, Richard