One of the 3 toilets in my house doesn't flush completely about 50% of the
time. Usually, when the toilet doesn't flush completely, any solids will
still be removed but the water just kinds of swirls around without
completely flushing. If I reflush (when tank refills) the second time will
almost always work OK. Sometimes the first flush will work just fine.

The toilet has been in the house since it was built in the 70s. The toilet
is not the newer type that uses less water.

I have tried a plunger, but I don't think the line is blocked. Shower and
sink in the same bathroom work OK.

I have had this problem on and off for years but I've just lived with it.
Lately, it seems to be getting a little worse.

Thanks for any advice.