I'm sure that there are other possibilities, but if you have hard water, that toilet may get used more than the others, and you have some mineral deposits in the jet hole (this assumes it HAS a jet hole!). That can slow the start of the siphon when you flush. You might want to take a wire coat hanger and ream out the under rim holes to see if any of them are clogged. That can also slow the water flow.
Does this toilet use a float on the flapper? Sometimes, those things come loose and actually get flushed down into the toilet, and they won't allow the water to attain the normal velocity, which can affect the water jet that starts the flush.
Something as simple as flushing some floss down the toilet, if it gets caught just so, and act like a wick and lower the bowl level. If the toilet does not sit level, that can affect the bowl water level slightly, and may be enough to cause problems.
Older low-flush toilets were not all created equal, and some of them were just dogs. THrow in less than stellar quality control, and there may be some other internal issue with the toilet.