There is no limit in the module, afaik. Try to see if some gateways are in no probing mode via rpc command. Also, be sure you capture all the network interfaces or enable event_route[tm:local-request] and add there some log messages to see if local OPTIONS requests are sent.
You should upgrade to latest 5.2.x to be sure is not a side effect of some fixed bug since 5.2.0.