At her age she still could be a candidate for possible coccidiosis. That can cause a crop slowdown. Symptoms of coccidiosis are sleepiness, standing around hunched with the neck pulled in, runny poos sometimes may have blood or mucus, poor appetite, and weakness. At this point if she has any symptoms, I might go to feed store or TSC to get some Corid (amprollium) and treat her for 5-7 days. Dosage is 1 1/2 tsp of the powder or 2 tsp of the liquid per gallon of water. It is safe for the other birds to drink, or treat her alone. If you do Corid, stop the vitamins until after treatment, but make sure that she is drinking plenty.
http://www.thepoultrysite.com/publications/2/Coccidiosis Management/43/symptoms-and-diagnosis/
http://www.the-chicken-chick.com/2012/12/coccidiosis-what-backyard-chicken.html
http://www.thepoultrysite.com/publications/2/Coccidiosis Management/43/symptoms-and-diagnosis/
http://www.the-chicken-chick.com/2012/12/coccidiosis-what-backyard-chicken.html