Hi Wayne

A couple of possibilities that I can think of that may be coming into play.

Firstly, multidecoding is not as sensitive as decoding signals individually, so the signal that does not decode may just  simply  be below the threshold.  

Secondly,  if the signals are overlapping,  the one that decodes can  block out the other signal. 

Things to try are to attempt a manual decode of the signal that does not decode, possibly with a lower value of Ftol (btw Fol cannot be set to 0!).  You could try a value such as 50Hz, or maybe a little lower, but be careful to set the mouse cursor exactly on the lowest tone of the signal. Invoking a manual decode I believe uses Deep decoding automatically, which may gain you a little more sensitivity compared to Fast. Manual decoding is either by double clicking on the signal on the waterfall, or single click and press Decode button. 

If you were saving wave files, making these available would help further analysis.  

73

Charlie DL3WDG