It is probably just simply the case that Virgin have not released their Advance tickets for the Easter Bank Holiday Weekend yet. A number of TOCs are regularly later releasing tickets for Bank Holidays and weekends than they are for midweek.
I have also been waiting to book a ticket from Nottingham to London on Monday 25th April (Easter Monday), but EMT have not released their tickets for Fri, Sat, Sun and Mon of that weekend despite having released tickets for the following day Tues 26th April. EMT regularly do not release their weekend tickets until a good few weeks after the weekday tickets have been on sale.
East Coast released their AP tickets on Thursday and are now selling them right through until Saturday 30th April including all weekends and bank holidays. I find that this is usually the case with East Coast.
Therefore I have now booked a Nottingham to Kings Cross (changing at Peterborough) AP ticket for 25th April for only £9.95. Much cheaper than I would ever get by waiting for EMT to sell their tickets! I always end up tavelling to and from Nottingham to London at weekends and bank holidays with East Coast simply because they release their tickets first. I've made the mistake in the past of waiting weeks for EMT to release theirs only to find myself paying more than twice the price I could have got with East Coast.
EMT miss out because they don't release their tickets earlier!
I will probably end up travelling with East Coast for my return journey on May Day for the same reason.