If they weren't actually informed and invited to vote they could claim the IVA is null and void. However, if they were informed, they have no come back and are bound. It is their fault for not voting if they object.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk
If they were on your list of creditors but didn't bother voting - tough luck.
If they're not on your list of creditors and have been informed "after the fact" they have a period of 28 days from being informed to challenge it through their own legal processes. It's potentially not worth their trouble doing so, as if they don't challenge it and they end up coming into the arrangement and being a majority creditor, they may have the voting power to reject any increase in claims. Not all creditors will see it the same way though.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk