what I normally do at work changing a load of tubes and starters is replace the lot and if I have any bother with any take a tube and starter out of a n existing fitting u know works ok and try them in. if it works its a faulty new starter or tube if still giving bother its the fitting as said above some makes of starter don't always work
It might be related to the weather, florescent tubes don't like the cold. I had one in the garage & in the winter it would take ages to start. I cured it by fitting an electronic starter & I've used nothing else since.