Autoturtle always works well for me. Is it possible that yours expires before the units return to the haven?

I hit over 400 blocks yesterday and it worked for me.

@Ghost 

As Crowbar asked.. are you sure the auto-hide does not expire before troop return? .. remember.. if it takes 3 hours to reach the target - it WILL take 3 hours to come back to base.. so 6 hour round-trip.. in which case a 4 hour auto-hide won't work!!

Good example. Thank you, @Tids.

For anyone else reading, there are other things to consider too. For example, if you send your units to a trip, but then the global speedup or your own prestige expire before they hit, their return trip will be slower and thus take more time.

Even an edge case such as your Bastion getting downgraded while your troops are travelling can have a (minor) impact. There are many moving parts in this game and it's hard to consider all that are relevant.