Moscow Rule number 38: If an alert is issued, they must pay a price and so must you.
By raising an alert, the adversary shows their hand. In effect, they allow you to know what they knew about your operation. This is why police don't start arresting drug dealers as soon as they find them, but trace out their lines of supply and find out where the traffic is coming from. Arrests are then made all at once, to create maximum disruption to the criminal activities. To make arrests early would simply tip off the crooks that their operation has been compromised.
(From Wikipedia) The objective of counter-intelligence is to identify hostile intelligence operatives. Once identification has been made, the operative is not necessarily arrested immediately. A decision must be made, and hostile operatives can be left alone for a period of time so as not to reveal the existence of counter-intelligence operations. Nonetheless, an identified operative must be cut off from access to further secure information, without letting them know their cover is blown. They might then be given disinformation, either directly or indirectly. An assessment must be made as to the extent of the damage caused by that operative. Eventually, the operative and his handlers will realize their operation has been compromised if useless disinformation is being passed, but this creates time for the difficult process of "walking the dog backwards" to determine what has been compromised within the target organization. That is the point at which an arrest is usually made. Sometimes the process of feeding disinformation can be useful, and a hostile operative may be left in place for years. Occasionally attempts are made to "turn" a mole; that is, gain his cooperation without exposing to his controllers that his cover has been blown. Turning a mole can make him an unwilling agent of either side, either to continue the feed of disinformation, or being coerced at threat of imprisonment to betray his compatriot organization. In the famous case of Arkady Shevchenko, a Soviet diplomat to the United Nations who asked to defect, rather than accept his defection the CIA required he remain in place and engage in espionage. Shevchenko was a professional diplomat, not a spy, and he found the stressful work nerve-wracking.