I've had a similar problem. The whole Volt thing seems overly complicated and confusing, and really badly implemented, but it is a good offer.
I've managed to get my situation fixed by ringing Virgin, navigating the ridiculous multiple-choice phone system (I went for I had a broadband fault option....), when you get through explain you've joined Volt but your broadband speed hasn't been increased, the first line rep may try and fix this at the first level of customer service, this first line agent will probably fail, and will need to transfer you to retentions. You will likely have to wait on hold for a long time, once you are through to retentions they should be able to sort out your problem, you will need to explain it again, this absolutely should not involve you paying more.
I think these folks are hearing this a lot as the whole Volt implementation seems completely messed up, the main thing is you need to know is that you should be on the Volt breed of broadband at the speed you expect after your increase not the package you were on before applying the Volt benefit, confusing, right?
Very long story short, ring VM be prepared to calmly explain your situation a couple of times and sit on the phone a (long) while and they should sort it, perfect no, but at the end of the day they do fix it.