Brewinhard beat me to chase. The source of your problem is borderline aeration. Dry yeast does not require much in the way of aeration because it is propagated aerobically below the Crabtree threshold; therefore, it goes into a fermentation will with fully-charged ergosterol and unsaturated fatty acid (UFA) reserves. Cropped yeast is an entirely different animal. Cropped yeast has low ergosterol and UFA reserves. Yeast cells use O2
to synthesize these compounds, which, in turn, are used to repair and build plasma membranes. The plasma membrane is what controls how well a cell can pass nutrients and waste products through the cell wall. These reserves are shared with daughter cells during the fermentation. Low dissolved O2
coupled with quiescent yeast cells is a recipe for high terminal gravity.
If you have an old broken racking cane or length 3/8" copper tubing lying around, you can make a poor man's aerator by drilling a bunch holes downward at a 45-degree angle. The aerator is inserted into a piece of tubing that runs from your ball valve on your kettle to your fermentation vessel. Wort mixes with air when it passes through the holes. It is best to use this device in a non-drafty room. A shroud can be made by drilling a hole in the closed end of a prescription vial and inserting a rubber grommet with a 3/8" I.D. The closed end slides over the aerator, closed end up, such that the holes are inside of the prescription vial. Sterile cotton is lightly packed into the open end. The keyword here is lightly. Air needs to be able to pass through the cotton. The sterile cotton is there to collect dust. I do not use a shroud. I merely place the aerator down into a carboy. The air inside a freshly sanitized carboy should be relatively dust free, and a carboy is under positive pressure while it is being filled.
Poor Man's Aerator