Sounds like this toilet is not properly vented.
You should try the following
1- get a 5 gallon pail of water and dump it down this toilet to see if it takes full flow.
If it doesnt have your “plumber” remove this toilet THEN pour a 5 gallon pail of water down the lead bend (waste pipe) and see if it takes the full pail try this several times THIS will let you know if it is the drain line or the fixture.
Easy huh?