Like Derek I have never got on with freezer sprays, although I accept that they are probably ideal for daily maintenance.
Once the wad has been impacted into the carpet and mixed with grit, powdered glass etc. freezing just does not work. The fact that the wad will, on some carpets, contain carpet fibres means that if the wad is broken, these fibres will tear.
Citrus gel, aided by a good steam pistol removes most of the mass and a follow-up with d'limonene or 'Solvall' (used with caution, not poured on) is sometimes required.
John