do you use any performance-boosting autoexecs? check your console to see if your decals are disabled, can't remember the exact command so give me a moment
edit: see if r_drawdecals is set to 0, if that's the case then setting it to 1 should fix the issue
also might be worth checking your decal settings, r_decals and mp_decals. the default for r_decals is 2048 and mp_decals is 300 iirc