A Shih Tzu that is predetermined to be 9 lbs. and another that is destined to be 16 lbs. will both start off relatively the same size, but will gain weight on very different paths.
If you looked at a weight gain chart for puppies, the line showing weight progression will almost never be perfect.
Some will make gradual pushes on a seemingly balanced progression, but things will slow down in the final 2-3 months of growth and many Shih Tzu puppies will have lots of rapid gains, slow gains, and even some stalls.
From 8 weeks on, it is suggested to track your Shih Tzu puppy's weight every 2 weeks. There should, of course, be a gain. No matter whether that number is small or large, when you check again after the following two weeks, more than likely the gains will be different from each other.
From the 2 month mark to the 4 month mark, a stall should not last longer than 2 weeks. If you see a weight loss, this is another matter entirely, and should be reported to the vet asap.