That's not a normal symptom of one of their toilets. Most of their designs use a siphon jet (a hole near the bottom that points towards the outlet) to start the actual flushing action. IF the siphon jet is somehow blocked, it would revert to an older, washdown, gravity flush toilet (i.e., it would need to fill up lots more before there was enough pressure to start the flush).
Now, you could try this: after the toilet fills and the water stops, put some food coloring in the tank. Then, once it's evenly dispursed, look at the bottom of the toilet where the siphon jet is and see if there's a jet of colored water from the tank at the start of the flush. If there isn't, or it is a very weak flow there, that's the issue and you need to figure out what's going on. If you have really hard water, it could be mineral deposits, but something could be lodged in there somehow.
Right off, I can't think of anything else, assuming it always does empty. If it doesn't, the only other thing I can think of is that the drain line is backed up somehow...you cannot flush the toilet unless there's room in the drain for it to go!
