It is difficult to say what may be causing detachment of cells and whether the cells that are detaching are blood cells that are not being reprogrammed and are dying over long-term culture. If the cells that are detaching are pre-iPS cell-like colonies, I would re-examine the vector system and matrix, and whether the medium you are using is optimal for blood cells. We developed ReproTeSR specifically for reprogramming of blood-derived cells, and would therefore recommend that medium for your situation.