Stir it up real good, that should do the trick. Typically when using yeast from a previous batch you want to plan accordingly. Your first beer was big. You should work your way up to big beers. I.E. brew OG 1.045, stepped up to 1.065, then to 1.075, etc. not exact science, but your yeast is still viable and gets to grow with each batch. If pitching on a cake you may also want to look into washing your yeast too. Also check your gravity. It could be as simple as a loose seal and pressure is not building up enough to give u airlock activity.
Keg 1: Coal Miner's Daugter Porter
Keg 2: Red October
Keg 3: Hopsolutely
Bottled: 1 Vienna Lagers; Erins Irish Red; Pils, Christmas Doppel, Addy's Amber Ale(competition brew), Mayden Voyage Blonde Ale
Future: Something Belgian, ESB