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 

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