Poor you Eve how frustrating! I know I work for a much bigger rescue but this is how we work and I can't say we get that many people just not turning up for the animal, although I think there will always be some!
When they first come up to us we ask them to fill in a homefinder, then go through it with them, asking any extra questions we need to.
In the case of the small animals we then take them into the enclosures, go through all the details on the various rabbit/guinea-pig and ascertain whether it will be a suitable match.
If the person then likes the animal/s, we put the animal on reserve to them, ask them to bring up rest of family if necessary and then arrange a home visit. We then go out to do the home visit, go through everything about settling in, the animal etc. and then if all is OK arrange a time and day for them to pick the animal up.
How much do you charge for your rabbits? Could it be they are going elsewhere and getting them cheaper?
And also, it may help for you to actually reserve an individual to them, if they don't have anything on reserve then letting you down isn't such a big deal as they haven't fallen in love with anything!!
Also if they HAVE to take their rabbit to you for bonding, perhaps they think about it and realise they don't want their bunnies staying away from home? I must say that would most definitely put me off as I wouldn't want my bunny anywhere but at home ... and feel I could do the bonding myself. You say that that is the only way to be certain they don't go as single bunnies ... there is no sure-fire way as the people could split them when they get home! (sure they don't though! just trying to think of any barriers that may be stopping the adoptions going through!)
Hope that helps you!
Nicola