This may be due to the Mac BC and Windows BC being at different versions. You had best check this out with Garmin Product Support.
If your BC collection is not too large/complex, you could set up the same list folder and list structure on the PC for the missing recent data and export each list manually from the Mac to a .gpx and import this into the PC. This would be tedious if you have a lot of missing data ...
Your suggestions were very helpful! I really only needed a small number of routes transferred so doing it manually was not too big a chore. Of course it would have been better to just get all my routes transferred. I checked with Garmin support and was told the issue was not with the version difference and they had no fix for my problem - which was not too helpful.
