They don't get lost, they become stale, meaning the datestamp on the decimated wave file ("pictures" are actually copies of the clip with most of the samples removed) is older than the datestamp on the actual clip. Normally, this triggers an automatic recalculation, but that won't happen if the disk space used by all pictures has exceeded your configured maximum. Then, you have to either manually force a recalc or delete some pictures to make room. If you have plenty of disk space available, set the cache size to some large value, e.g. several gigabytes.