Depending on the heat and humidity silver coated and slightly buffered paper can darken spontaneously in less than an hour. It happened so I started using acid in printing. The silver citrate precipitate won't harm your solution but you have lost some amount of silver, it is no longer %11 percent.
Silver nitrate and citric acid can be mixed just before coating the paper. Into a shot glass for each 1ml of Silver nitrate add a drop of concentrated Citric Acid. I prefer to use %50, 20 to 40 percent is quite enough.