they will not repair if the nail is in the sidewall of the tyre,
or if it is a very large nail that may have done damage to the structural fibres of the tyre.
or if the tyre is worn so much or so unevenly that a repair could be dangerous,
but i have known tyre services to refuse a repair, simply to sell a new tyre,
I went to Kwik Fit with a nail in my tyre and all they wanted to do was sell me a new tyre. I got a mobile tyre repair company to call at my house, and he fixed it for 20 quid.
Try to find an independent small tyre company, they will not repair if in sidewall, but will repair "normal" puncture, steer well clear of places like KwikFit!