If a particular slave server has problems updating maps, log in to that server and run ypxfr interactively. If ypxfr fails, it tells you why it failed, and you can fix the problem. If ypxfr succeeds, but you suspect it has occasionally failed, create a log file to enable logging of messages. To create a log file, enter the following on the slave..
ypslave# cd /var/yp
ypslave# touch ypxfr.log
The output resembles the output ypxfr displays when run interactively, but each line in the log file is time stamped. (You might see unusual ordering in the time-stamps. That is okay—the time-stamp tells you when ypxfr started to run. If copies of ypxfr ran simultaneously but their work took differing amounts of time, they might actually write their summary status line to the log files in an order different from that which they were invoked.) Any pattern of intermittent failure shows up in the log.
When you have fixed the problem, turn off logging by removing the log file. If you forget to remove it, it continues to grow without limit.