Were you able to do the midiprep of this plasmid successfully in the past or is it the first time that you are working with this plasmid?
Did you have any problem to grow the culture? Which antibiotic are you using? Is it ampicillin? If it's ampicilin, try changing to carbenicilin.
I once had problems with a plasmid at the time of doing maxipreps: the culture growed very well, but after the preparation there was no DNA or a very very little and useless amount.
My plasmid contained a gene for ampicilin resistance, so I solved the problem just by using carbenicilin instead of ampicilin. On some occasions, when plasmids are "toxic" for the baceria, bacteria express ß-lactamase that degrades ampicilin in the culture and this way they can get rid of the plasmid, because they don't need it any more, and start growing without it. This way, when you do your midi/maxiprep you get hardly no DNA, if any. Well, more or less I think that's the way it works. Carbenicilin is less easily degraded an therefore bacteria should conserve the plasmid in order to grow.
Just an idea.