This is, as others have said, due to the government not being clear around the furlough arrangements. When furlough was announced on March 20, a lot of nurseries, like other employers, assumed they would be covered.
However, some three weeks after Furlough had begun, the Govt announced that companies that already paid their staff through government funds (like nurseries paying staff using the funded hours) could not claim Furlough. Many will have already done so, so need to pay the money back, and they will need to (and are entitled to) use the 30 hours funding to do so. Not doing so would involve the Government essentially paying the salary twice.
The problem lies with the communication with the parents. Essentially, the arrangement you have with them to use the hours continued whilst on lockdown; the funded hours continued to be paid, you just couldn't use them, unfortunately.
The issue you specifically now have is that because you normally "spread" them over the year, there are no more hours left for you to use, as the nursery was entitled to use the maximum 30 hours a month during lockdown. This is because that is how the Government pay them - the nursery allow you to spread them, but that's really just an accounting issue. They don't have to offer to spread the hours.
It's unfortunate, and I suspect no-one in Govt thought about the impact on those who normally spread their hours over 52 rather than 38 weeks.