Were you by any chance checking with Dscan to see if there were other players within a close proximity?
It’s quite possible you jumped in while other players were already clearing the rooms. If you check out the Eve University page for Combat Sites you can check which one you’ve found. Combat sites - EVE University Wiki
Some of them have locked acceleration gates and some of them don’t.
Also, not to discourage you, but 30 minutes is a long time to clear a room. There is a lot of competition for sites in high traffic regions. You’re far better going off the beaten track. You can also blitz anomalies for escalations, but I’ve never found that particularly productive myself and I get bored pretty quickly doing it.
Edit: Also, in case no one has warned you; If someone warps into a site, loots your stuff and goes suspect… Do not attack them as a new player. Especially if they loot something that is menial. They’re baiting you so they can attack back without Concord attacking them.