Reduced water in toilet U bend ?

I have 3 toilets (loos) in my house, 2 upstairs and 1 downstairs. The water level in the toilet bowl in ONE of the upstairs toilets has been significantly dropping between flushes for the last month - it never used to do this and none of the others is doing it.

I looked and found a previous answer which said this is caused by the wind blowing over the stench pipe up the side of the house and creating a 'venturi' effect. This 'suction' air pressure sips a little water over the U bend in the toilet pan, which goes down the waste pipe. Hence, less water until the loo is flushed. Another previous answer said this would affect all toilets connected to the waste system. All of mine feed into the same sewer and we only have one stench pipe.

So - my question is - why is this only happening with one of the toilets and should I call in a plumber ???

Thanks for your time if you are able to help.