There are only 2 reasons log data would be skipped:
1) The UOW was aborted
2) The timestamp on the log records are outside the processing time frame.
The latter case can occur because of timestamp inconsistency. If the processing time frame is 12:00 to 14:00 and a log selected for processing has a RECON start/stop time of 12:00 to 12:30 and contains log data stamped 11:00, then this log data will be skipped. This problem can be avoided by setting EXPANDTIME=60 to accommodate the 1 hour lag time in the log record timestamp. Doing so would have caused the previous LogExtract run to select the 12:00 to 12:30 log from the RECON dataset with the previous processing time frame (say 10:00 to 12:00) so that the 11:00 stamped log records would have been extracted.