Glad to be of service!
As for the issue at hand, it probably has not been done due to costs.
Technically speaking, sure, a suit of power armor can be made to be completely invisible to radar detection. However, it's going to cost you a boatload of cash to do it. There are even a couple of different ways you can do it. You can go the route of RAM, angled design work, and composite construction. You could even go for an electronic spoofer system, like the active stealth system outfitted on the French Rafale.
Generally speaking, if you want something that is stealthed, make it an aircraft if you want it to fly high enough to be picked up on radar. If you want it to be able to be in water - make it a boat/ship or better yet, a submarine. If you NEED it to be power armor though, have her hug the ground to steer clear of detection.
If you still feel you need stealth power armor, let me know. I've got a power armor creation table that includes stealth as an option.