Winston And Julia's Rebellion

After a while of sneaking around together, Winston and Julia finally decide to just keep meeting at Mr. Charrington's apartment. They keep meeting there for a while too. However, it is later revealed that a telescreen is hidden behind the painting. Also, Winston and Julia meet with O'Brien, who is later revealed to be working for Big Brother. With all these thing to give them away, why weren't they taken to the Ministry of Love sooner? Why would the Thought police wait so long to apprehend them?