Any bowl can be restricted if something is lodged in the trapway. A clean trapway should siphon well. Sometimes we need to run an auger through, to clear them out.
It can also be a semi-blocked siphon jet hole.
Have read so much about these that when offered a free one, I called AS for the new flush valve, installed that, and set the toilet at a low-use location. Tank 4260, date March 2005.
I don't see any mechanical problems but it just doesn't seem to flush consistently. Sometimes it barely seems to siphon and needs a second flush to clear the bowl. Not exactly a "champion" in performance. I looked the bowl over carefully before setting it but didn't see any problems, but I know there can be an internal defect or obstruction in the passage.
If the toilet truly only sees occasional use, some water from the bowl may be evaporating, lowering it enough so it doesn't flush well. Any time you try to start a flush when the bowl isn't full, it can result is poor performance. But, it may just be it isn't that great of a toilet. When the bowl isn't full, the first flush is weak, but the refill leaves the bowl at the 'proper' level...then, the second flush works as normal. This isn't normally a problem (assuming it is evaporation) when the toilet is used often enough so the level isn't compromised. I notice my toilets bowl level is low if I'm away for a couple of weeks or more when I return. Do not notice that at all under 'normal' circumstances when they get used on a daily basis.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013