Signing in from different Macs, inconsistent purchase history

I'm about to do a clean install of Yosemite on one Mac so am checking that all my app purchases will be downloadable again. When I sign into App Store using the same Apple ID on another Mac ('Mac B'), at least one app that I purchased on Mac A still shows as not purchased (button shows the price) on Mac B (even though it's installed). Other apps do show as purchased – button shows 'Open'. Any ideas what could deb going on here? Thanks all!