I tested both my mother "M" and her brother "D" at FTDNA. I was looking through D's list of segments today for triangulated segments, using the matrix utility to help, and I found the following
D matches Person A on chromosome 1 from positions 55980120 to 62971860
D matches Person B on chromosome 1 from positions 55980120 to 69360781
D matches his sister M (my mother) from positions 54879395 to 81490003
According to the matrix utility, M is also a match to A and B but A and B do not match each other. At first, after some thought I decided that D and M must be full matches on the relevant segment (i.e., they match on both of their chromosomes), and A matched one chromosome while B matched the other. But using David Pike's utilities and double checking with the graphic at GEDmatch, it is clear they are only a half-match.
Then I thought maybe the match between A and B existed but was too weak for FTDNA to consider them a match. But their overlap is over 10cM and is over the threshold that FTDNA uses (which I found by Googling).
So I am at a loss as to how this can happen. Any ideas?