I am having fantastic results with my kitty who has constipation issues and a canned food called Merrick Before Grain. It's not cheap but she is doing so well on it I just am amazed.
Before she was getting Wellness canned grain free, three flavors, beef and chicken, chicken, and turkey. After the recent wellness recall I realized I needed to vary her diet more just in case, so I started her with the Before Grain. I feed the Turkey, beef and the chicken (I never give my cats fishy foods)
She is on about half and half now, half Wellness, half Before Grain, but there is a marked improvement in her energy level, coat quality and ease of pooping recently, I started the BG about 6 weeks ago.
Cats with constipation do best on a grain free diet. And they need a LOT of water.
I add water to every meal I feed her, I feed small meals, about a quarter can at a time with a couple tablespoons of warm water added to it. I cut the food into small chunks.