There has been a lot of expert posts about acoustic properties through water. Salinity, depth, temperature layers etc..
Added to this, what about the geography of the ocean floor?
If the wreckage sits within a valley then this will certainly make it harder to detect the cvr/fdr pings?
Could this be the reason that the 2+ hours capture time with Ocean Shield happened when the towed pinger locator wasn't at its full depth - so was possibly more likely to pick up a signal for a longer period of time?
I.e less affected by peaks and troughs of the ocean floor..