Unbeknownst to most people, collecting the blow off from a fermenting beer is not the same as top cropping and is not good brewing practice. Real top cropping involves removing the first 'dirt' or krausen - which contains weak or dead yeast, trub, and bitter compounds - and then waiting until the second (high) krausen to harvest. Ideally, you want to wait until the beer is just over 50% attenuated. This ensures you are harvesting the cleanest and most active yeast that share the same characteristics of the first generation.
With that said, wy1968 is not a good yeast for top cropping as it flocculates too fast and doesn't retain a top layer of yeast. Your best bet would be to wait until the fermentation is done to repitch some slurry or use another yeast.