I agree with the sage advice above re the need to keep sand out of the join. You say you are doing this, anyhow.
However, at the risk of asking a stupid question: Is the mast top and bottom a matching pair (e.g. same brand, same model and same material)? Having joined your two mast sections together, can you then
immediately separate them, or do they bind, regardless?
Since you say you are new to windsurfing, you may think that any top and bottom mast section will fit and work together. Sadly, this is not the case.
You may be able to relieve any binding with
an angle grinder light sanding with wet and dry sandpaper.