Have you tried taking heavy mineral oil (USP grade for oral use)? All other OTC laxatives did not have an appreciable effect upon my problem. But 2 table spoons (minimum of 2, probably a little more depending on your constipation severity) before bedtime completely alleviated my constipation, which occasionally was full on impaction. Most severe, once, took 3-4 hours to pass, with horrible pain/stressing. A single solid piece the size of an average spray pain can(actually a bit larger!) ejected, along with plenty of fresh blood as it caused some tearing, etc.. Even the saline application with the long tube up your anus did not clear me. After this experience, I do take at least a little bit of mineral oil every other night before bed, or every night, if I began to constipate a bit. I have had no impaction since starting to use the mineral oil.
Don't use mineral oil before a meal, or even shortly after a meal. There seems to be potential for the oil to block absorption of some vitamins/nutrients if you use it before a meal or soon afterwards. Maybe use every other day has some negative potential if used on regular basis in the long term. Perhaps consulting a doctor about use of mineral oil if you plan on using in the long term is a wise move.
Before someone suggests I my have a lack of fiber in my diet: it is not unusual for me to consume 50 or more grams a day of dietary fiber.