In a world with magic that regenerates over time, such that taking a short break ½-2 hours long is enough for high magic individuals to restore a portion of power (and cast more if they were originally out of magic) and taking a much longer break 8+ hours is generally enough to restore all of your magic. The Mages of this world are able to create magic items to hold the power of a spell such as scrolls, wands, staves, and all manner of non-spellcasting items (swords with fire enchantments etc.).
However, when they use magic to create these items using the requisite quality of materials for holding it, the power they used to create them is not regenerated over time despite being expended. What would prevent them from regaining power?
Edit: This effect remains even for items such as single use scrolls until the scroll is used.