In what way have they lost their mojo? If they're leaking then there isn't much you can do, the tech wash type treatments are to cause the water to bead so it doesn't wet out, they're not designed to create a impenetrable barrier and beaded water is still going to find it's way through if the material isn't waterproof, though it might slow progress. It's unusual for simple none breathable nylon to lose it's waterproof, have you tested it, simple to do, just make a hollow and fill it with water.
With more expensive breathable fabrics, the water repellent sticks better to any that already exists, so you have a choice between a garment clean enough to maximise breathability and a better chance of reproofing... It's a common experience that it works but not for long, it simply hasn't bonded well to the cleaned fabric. I doubt there is a correct answer, wetting out is still waterproof unless that layer is knackered, I had a Gortex jacket that I was convinced let water through, but a test showed otherwise. I wash my current E-vent jacket in a very weak soap and spray on TX while it's still damp, followed by a hairdryer to dry off. It's working reasonably well, though the jacket is grubby and I fear a proper clean will ruin it.
EDIT - in the days before fancy fabrics (Or more likely my ability to afford them) I used to work long days outside and we'd spray jackets, overtrousers and boots with some stuff intended for buildings, it smelt for a bit, then worked fine, even on stuff that wasn't waterproof to start with. Thompsons weather seal looks to be the same stuff, though that wasn't it.
