When jackets get dirty and tired, they no longer 'bead' rainwater and will 'wet out' easier - This is when the outer layer becomes saturated with water.
They should still be waterproof, because the inner membrane isn't affected by this. It's 100% waterproof, regardless.
However, they won't feel waterproof, because the breathability relies on the water-prepellent properties of the outer layer, which is now saturated and so won't pass as much air. So, any perspiration or moisture from your body has nowhere to go, and you'll feel wet - even though the water's not getting in from the outside.
Anyhow, have you read eVent's care instructions?
Always follow the garment manufacturer’s care and use instructions.
Below are general considerations for maintaining garments with eVent
fabrics. Avoid heavy soiling, which could affect membrane performance
temporarily. Thorough washing will generally maintain and restore
product performance. Regular washing of your eVent garment will
enhance product performance. Your garment may be machine washed in
warm water (110F / 40C) with liquid detergent, and we recommend a
second rinse to remove all traces of detergent from the fabric. We
recommend you drip-dry your garment. Remember also to refer to the
garment manufacturer’s guidelines. Do not use fabric softeners or
bleach. Never dry-clean your garment. We also recommend that you avoid
ironing the garment, but if you do, use the steam setting on no higher
temperature than “warm”. At some temperatures, the heat from the iron
can have a rejuvenating effect on the DWR finish but there is a
greater potential for fabric damage if the iron is too hot.
- Warm wash with liquid detergent.
- Second rinse to remove any lingering detergent
- Warm Iron (under a tea-towel?) to rejuvenate the DWR
I've heard that leaving the jacket in a car on a hot day, or carefully using a hairdryer on it will improve the DWR without the risks involved in ironing.