Hi, Jay.
It may be that your trace interval is more frequent on that target, so the default maximum 250,000 samples in memory is reached more quickly on that target.
Or ... the "Maximum samples in memory" may be set differently for the configuration for that target. This is in the "Auto-save" section of Edit -> Options. If 8 of your targets are using one configuration and 1 is using another, that "another" configuration might have a different setting for this.
Note that you shouldn't have "Unlimited" for any of your targets, or you will run out of memory at some point. See here for details:
http://www.pingplotter.com/manual/pro/long_term_monitoring_and_auto_.html