Help me understand this

Honestly I hate whingers but here I go.
I put an overs multiple on this morning and since then the Rotherham game has been postponed.
I call betfair and ask if I can put in another match to replace it but they say no!
Can anyone explain to me why this is as
a) There is still an hour before any of my matches kick offs
b) On multiples you dont play against other customers
c) Had the same match on my '2 teams must score' bet with Fred and I got it changed there without any problem.