AS Cadet flush problems

[ridler72]

I installed an AS Cadet 3 toilet in my powder room about a year ago and have had inconsistent flushing ever since. Most of the time when I flush the toilet the water swirls a little and fills the bowl then slowly drains to a normal level. But, if I take a bucket of water and pour it into the toilet it flushes as a toilet should. Any idea what the problem is?

I've adjust the chain to ensure the flapper is pulled all the way up and ensured the tank is filling properly as well.

Not sure if this has anything to do with it but the house was built in 1965.

Thanks for any help.

Ridler