What usually happens when an allograft does not take in the long run due to advanced gum recession specific to lower teeth due to bruxism?

Graft failure..: (i believe you may mean auto-graft) the 'gum' graft may lose pink color and slough off like peeling paint from a painted-surface. Talk you your doctor!

Allograft Failure: The purpose of grafting, whether an allograft or other donor source, is to restore the supporting architecture as much as possible to it's original ; health state. Anything less than that increases your risk of periodontal breakdown. Lower anterior teeth in particular have thin roots, where calculus tends for form more ; are more prone to the ravages of gum ; bone loss. Discuss with periodontist.

Graft Should Take: If a proper history and clinical examination is taken, a graft, whether soft or hard tissue, should take just fine. There may be additional Tx indicated, especially in cases of bruxism or clenching. Also, any aggressive brushing of the grafted area is contraindicated. If anything, the graft is being done to correct the recession, so it should work based on the prognosis given before surgery.

Second graft: The failure of a dental allograft usually is because of an infection or because the grafted bone was unstable and not because bruxism. If a dental allograft fails, it is removed and a second graft can be placed once the area has had a chance to heal.