For what it's worth: as far as I remember, in the previous CMs, the snipers worked well if you let them choose their own targets (i.e. you didn't need to manually give them targets). I remember a CMBB (or was it CMBO?) thread about the snipers who were reluctant to fire. It was mainly due to the fact that their "behaviour" was coded so as to give survival a high value (-> they try to keep a low profile) and they were very (!) reluctant to open fire when the enemy was too close.
Now, this is CM:SF, so I don't know if the coded behaviour is the same, but I would try to do the same as I did for CMBB: find a nice position and let them the initiative. May work. At least that's what I do in CMSF and so far it seems to be OK, but then since I play RT its sometimes difficult to be sure they fired, so I guess I'll have to check the ammo count