You do not mention what buffer you are trying to do your cleavage in. You need a reducing agent for TEV to work (e.g. reduced gluthionine, DTT, mercaptoethanol). EDTA (0.5-1mM) is recommended as TEV is a cysteine protease and the presence of divalent metal ions will/eventually inhibit TEV. TEV becomes less active as salt concentration increases (50% active at 0.5M NaCl)
If your protein contains disulphide bridges and you want to keep them intact you can use a ratio of 3 mM glutathione/0.3 mM oxidized glutathione which provides enough reducing power for TEV to work but should not disrupt disulphides. If none of these reasons are why your protein fails to cleave, there is a strong possibility that the TEV site is inaccessible. You could try 1M urea in this case. TEV again will be less active, but you could at least test this theory.
