I've had pretty good luck refreshing the DWR using:
1. Atsko Sport-Wash, followed by:
2. Atsko Fluoropolymer Permanent Water-Guard (in aerosol can).
It is (supposedly) important to first clean out your washing machine of all detergent residues by running a water-only cycle before washing the garment with the Sport-Wash.
After washing, dry in the dryer normally (make sure it doesn't get too hot to melt it). At that point you can do the "run the arm under the faucet" test to check the performance of the remaining DWR. If water doesn't bead up and roll off nicely, then proceed to the Water-Guard step. Completely wet-out the garmet with the Water-Guard, then allow to air dry. Then you must activate the DWR with heat. You can do it in your dryer if it gets hot enough, but I've found that mine does not. I use an iron set on low, and of course you want to run the iron over a similar test fabric to make sure you're not going to melt anything. After ironing, I do the arm under the faucet test again to verify that the DWR is good.
This is all a rather labor-intensive proceedure, but it's worked for me.
As mentioned, a DWR coating is not the same as waterproofing. But if rain beads up and rolls off, the actual "waterproof/breathable" layer has a much better chance of working as intended.