Anybody else having wifi issues with Spigen Slim Armor Cover?

Since I bought this cover, the Spigen Slim Armor, I am having wifi issues with my iPhone 5 running 6.0.2. The phone is a refurbished one I got as a replacement at the Apple Store and it came with 6.0.2 on so I can't know if it had issues before the fix. The first week I didn't notice any problem and it seemed fine just like my previous iPhone 5 that I returned for power button problems. Then the Slim(?) armor came which I assure you is not slim at all and I began to notice a fluctuating wifi signal resulting in an average loss of one signal bar. The bar keeps moving from one to three bars and in places where I used to get a solid three bars signal now it gets two. In borderline spots I barely get a single bar. It feels like the phone lost sensitivity to wifi. My question is if anybody else suffered the same issue and whether is is possible or not that a cover (even if thick and dual layered with solid plastic like the Spigen) can really affect a signal which is supposed to travel through concrete walls. Or should I swap this phone with another one at the Apple store...entering the infernal circle of finding a new phone with: working power and home buttons, no scratches or dents, working proximity sensor, working wifi chip. Every time is just a question if luck...