No, the wattage is immaterial, just means one will be brighter than the other.
if your Christmas (and b*llocks to the PC brigade) lights are of the type where all bulbs need to work, try switching places the new bulbs in positions where the original bulbs work.
if they are of the type where they all come on irrespective of whther there are bulbs missing try the 7w bulbs in places where they normally light to check if they work ok.
When you put them in and switched on I guess they didn't light immediately? leave them on for a few seconds in case they are flasher bulbs and need to heat up.
Failing all of that, chuck the lot and buy a new set