Being noticed by security services doesn't stop you carrying out at attack in democratic, open nations, as several recent cases have proved. There are too many radicals for the security services to watch properly and with encypted comms/darknet it's damned hard to stop them communicating with each other or to monitor what they are planning. We don't have enough agents or money to physically watch 1000's of people.
Security services are apparently thwarting plenty of attacks but plenty will get through. There is no way of stopping it and several thousand young men due to come back from these fighting zones in the next couple of years is going to ramp up activity hugely. IMO, this is the tip of the iceberg and it's going to get a lot worse.