Probably your target had recently been attacked multiple times - likely by artillery barrages (ABs) or guerrila strikes (gs). As a country suffers hits the return to an attacker diminishes (dr). Dr fades over time although activity by the country (expending turns and, particularly, making hits of its own) speeds up the rate at which dr re-sets. After three days any hits have ceased to have effect (that period is different server to server).

What you do is this. When you identify a likely target you do a search against its name or number in the news. If you see that no one has attacked it recently you know you won't have the experience you had with this guy. But if you see a whole string of hits on him/her you go elsewhere.

The first three hits over a short time yield well, the fourth hit and you see dr starting to cut in. Look in the news. You'll maybe see someone doing a series of SS hits (part of a landkill). Look at the first dozen hits and you'll see dr in action.