You are a "victim" of a game mechanic called Diminishing Returns (DR). Basically, the more attacks a country suffers in the last 72 hours, the less damaging subsequent attacks will be. DR only applies to PS and SS attack types (as in, only the returns of SS and PS will be reduced by DR), but all non-missile attack types will add to DR.

DR decays over time using the standard half-life formula for decay, on Primary this is 18 hours multiplied by the amount of attacking power sent divided by the defenses of that attack (if you sent 110% of his defenses, the halflife is 19.8 hours). Attacks older than 72 hours are ignored.

Attacks made by a country reduce that country's DR in the same manner (same halflife formula). For SS/PS, all attacks reduce DR, for other attack types (BR/AB/GS), only successful attacks reduce DR. Missiles do not affect DR in any way.