Do you have a stone at the end of the air line? If so, take the airstone off. You still need to shake up your bottle to keep it in suspension. When I was growing phyto, I'd give a gentle back and forth once a day to keep stuff off the bottom.
The clumps are getting bigger because the phyto that is growing in the water column and is alive is eventually settling on the bottom, piling up on the stuff that is already there. The clumpy stuff is probably not alive.