Did you use/install a 1/2" ball valve and then use a 3/8" barb fitting in it? All my valves, fittings, and hoses are 1/2" (ID for valves and hoses). I use silicone tubing between the keggle and plate chiller, then back again to the keggle (recirculate) before running into the fermenting vessel (also the same 1/2" ID silicone tubing). Using the same tubing makes it easier to put it all together since you don't need to keep the low temp side separate from the high temp side.
Even if the OP uses an IC, I think silicone tubing is the better route to take. It will last a loooong time and you can boil it if it gets nasty inside (if you don't clean it well after use). Plus, it tends to be more flexible than the vinyl tubing of the same size.