I've bought 'H' and 'B' class semi-flex fares (on domestics) before and not been allowed to change on the day, despite there being plenty of available seats. BA were only selling 'R' fares on the day, which I must say always seems to be the case. So, if 'H' and 'B' domestic fares don't seem to be available on the day, how exactly is one supposed to utilise the benefit of a semi-flex ticket? Is it all just a big con?
Now that is interesting. I didn't know that the same selling class has to be available to changed semi-flex tickets on the day, and am still not sure that that's correct. Were you perhaps trying to change within the hour as above, for which the rule isn't overly easy to spot?
They're not the type of tickets that suit my needs as a general stand, but if the rules are adhered to correctly by the traveller and agents alike, I think they represent quite good value for on the day flexibility compared to the other fully flexible options (and H/B offer full TP's too)? I wouldn't describe them as a con, just a bit more digging around on the rules is needed before purchase/transfer on all sides.