Hi,
I know. It happens whenever the requested atmospheric data's cycle has no corresponding wave data cycle i.e 06z when wave has only 00z and 12z.
The option to fix are:
1 create 4 cycles of wave data. -- a bit heavy on downloading/preprocessing/storage with current resources
2. fix server software when no matching cycle. -- options are to use wave data starting from previous cycle or extract wave data starting from atmosphere data cycle. The advantage of the fix is that it also provides a fallback if a particular cycle of wave data is missing from regular production.