Are you by chance using ACE3?
If so try this: go into your ACE Settings and disable the "Advanced Missile Guidance" setting, for both the Pilot and the Laserdesignation unit. May not be it, but it's worth trying out.
Also make sure that you do have batteries for it as mentioned by the poster above.
The GBU-12 Bombs are indeed laserguided, and atleast for me, they work without any problems (Worth noting, I use my own TGT system, so your results may differ).
Also can you confirm that you actually have Line of Sight to the exact spot the Laserdesignation unit is aiming for?