Nexus 7 cant connect to WIFI [Saved , secured with WPA/WPA2]

STATUS : when connecting to the wifi (connecting.....) the signal icon decreased to 1 which is weird since before connecting it is was full ....

After Updating it to Lollipop, after some time it suddenly began not to be able to connect to the WIFI which is unusual .... Occasionally I can connect to the wifi by luck ,by turning off the wifi router and the nexus itself

I tried
1.factory format
2.flash format
3.settings adjustment
4. I even downgraded it to Kitkat (4.2.2)

Need help :(

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