This year I purchased a brand new Besson Tuba for my middle school band program. However, I have noticed since I switched a student to this tuba, they consistently play flat. This student never had the issue before when they were playing on a Jupiter tuba, so I'm starting to wonder whether the issue is the instrument. The instrument is a Besson Tuba BE 187.

Does anyone know if the Besson Tuba BE 187 model has issues with playing consistently flat?
If so, how do I fix this issue?

If not, how do I help the student play in tune besides telling them to firm up their embouchure a bit and put more air through the tuba?