Maybe it's because those replays were played with an older version of WA?
Not an older one... All replays with versions that are not newer than the version of WA you use are playable.
Probably the file associations are messed up. Right-click the replay, open Properties, then, near the application field, press "Change" and make sure the newest version is used.
If those replays were saved with a PX scheme in use, you can't open them without PX.