Are there any rules?
A buy plan will be removed if it has failed to place an order three times in a row. The user will receive an email when this happens, but no warning beforehand as of yet. Any error will be added to your log, however, so be sure to check it regularly on the Plans page.
A buy plan may fail to place an order due to insufficient funds, no API keys uploaded, invalid volume, etc. Sometime Kraken's service is simply unavailable.