If her compamy's holiday year startes in april, as many do, then if she has booked a holiday early in the year, within the first 3 months of the financial year for instance, she will not have accumulated enough holiday days, for that reason some firms don't allow it. Thing is though, using that system, holidays end up getting backed up and too many people are left with too few months when holidays can be taken to suit the business, it's better to let staff take their holidays sprewad out throughout the year, then if they should leave during the year the holiday pay is deducted from final salary