Pull the deck and inspect all of the mounting parts in particular the double bent pin
Inspect the holes for ovaling
In use the left side tends to lift because of the belt rotation , and this is common to all belt driven decks .
And has already been mentioned try backing all of the adjusters off to their mid positions then level the deck from scratch.
Remember to both LOWER the high side as well as raising the low side.
Check where the hanger brackets are connected to the deck often they will bend , stretch or wear .
It is not uncommon to find the deck itself to be bent up where the hanger is fastened
Usually this is worse with big decks where the customer steps onto the deck when mounting the mower .
To be Zoom specific, the only one I work on is a mini Zoom and it required a lot of work to get it to lift square .
Despite the lift shafts being bushed , all 4 holes were oval and each had a different shape
The holes on the brackets welded to the deck were also oval and again each one was a different shape
The two drag links were badly worn as was the holes they sit in .
It is a very robust deck mount but it does require regular lubrication which this customer ( and probably most of them ) never did .
It was a big job but a lot of building up and welding on some high tensile washers + replacing the 4 bushes plus both drag links did the trick .
Down side was the two seasons later they ran it along a slope & siezed the engine