Well, I feel sheepish admitting this, but here goes: We've had some algae lately and I've been shocking but not brushing every day. I'm 7 months pregnant and my husband is extremely busy, so I've just let it go. Well, I ALMOST passed the OCLT this morning (missed it by .5). This afternoon I decided to just do the best I could with brushing because even though we almost passed, the algae was still on the walls of the pool. I brushed what I could and added a decent bit of bleach. After testing the water not long after I saw quite a loss of chlorine - more than to be expected after adding the bleach I did. So I added more and had my husband brush the deep end (the part I can't do with a big ol' belly!) really good. Did my final test tonight for the OCLT process and saw the same thing - the bleach I added to get us up to our shock level didn't do the trick. I've decided to skip the OCLT tonight, added enouch bleach to bring us to shock level and tomorrow I'll brush the pool more and do the OCLT tomorrow night.
Point of my writing this is I think I've learned. I read in another post somewhere that the algae builds up a protective layer against chlorine when it's on the walls of the pool. My husband was getting disheartened about the BBB method due to us still seeing algae in the pool...but now I think we both "get it" - that if we don't brush, the bleach added essentially does nothing to the algae, and that's why the algae wasn't "going away," and also why we were able to almost pass the OCLT when still seeing algae in the pool. The algae sitting on the walls doesn't seem to affect the chlorine levels, while the algae that is brushed into the pool eats up the chlorine because it's then that the chlorine can kill the algae, thus affecting chlorine levels.
If I'm still not getting it properly, please let me know. Otherwise, I'm glad to have learned this (even though the hard way) and will plan to brush the pool to the best of my ability when my husband isn't able to, and then get him to do a better job whenever he is free. Why isn't there a robot type of thing we can buy to brush the pool? That'd be nice!