Help with Ficoll gradient

Hi everyone, I work in a lab that often isolates human PBMCs. I dilute the blood 1:1 in HBSS (no Ca, Mg, or phenol red) and then layer it on top of 15mL Ficoll (or sometimes Lymphoprep).

My initial layering is usually fine. But sometimes after I’ve added 15-20mL blood on top, red clumps (probably RBCs) begin to sediment below to separation line. This also happens while finished tubes are sitting while I finish other tubes before centrifugation.

Does anyone have tips on how to prevent this? I think it is making my PBMC layer not separate well, and affecting my yields. I’ve done this a bunch of times now and can’t seem to avoid this, when my lab mates seem to have no problem.

Any tips would be greatly appreciated 🙏