Wyeast 1968 is a highly flocculant strain, which is awesome for beer clarity, but not so awesome if it clears before completing fermentation.
I had similar issues with this strain, so I started slowly increasing the temperature after high krausen (1 deg F per day) - basically a diacetyl rest.
Slowly increasing temp. after activity peaks motivates the yeast to stay in suspension and complete fermentation.
Mort's on point - if temperature isn't controlled, temp will ramp up quickly then the yeast will stall and floc as it cools.