Sometimes when releasing a slot, in order to "find" it again, you've got to change you departure time by +-5 minutes. No idea why this happens, but it often does.
This is because the way it's calculated. .. The airport has a capacity of let's say 60 movements / hr => 5 movements for every 5 minutes (slots are counted in 5 min blocks).
Each DAY has the max number of movements, and each HOUR has the defined max, and so does each 5 min block. But the 5 min blocks and hour max values (60 and 5 here) can flex a littlebit to allow overdraft - but in such a way that it never exceeds the daily capacity. I mean that if the 5 min capacity is 5 movements, you can actually have 8 movements at 08.00, and then 4 movements at 08.05 and so on, and not just 5-5-5-5- for every 5min block .. but the overall totals for hour/day cannot be exceeded and thus after closing a route you may not find the slot at the exact same time anymore as the overdraft limit has become full already and you need to adjust the deptimes a bit.
But if you have the slot (didn't release it when you closed the route) the same deptime should always work.