Use the following two commands from CLI to increase the HA timeout on your XenServer, first you have to disbale HA then re-enable HA via the CLI with the timeout switch. I've found that 120 seconds works best but it all depends on your environment and what suits your needs best.
xe pool-ha-enable heartbeat-sr-uuids=ad1983ed-66cb-8fd7-1ef5-66a41c9d4345 ha-config:timeout=XX
Obviously change the SR UUID to the UUID of your own SR and where =XX change this to the number of seconds to wait until HA kicks in by default, it is 30 seconds.
NOTE: to get your list of SR UUID's type xe sr-list in the console