What is the longest time you let it sit at the logo?
What model number does your phone have?
The flash you attempted, was it lower or higher than what was on the phone already? You may have to flash it with the latest Android, then side load the latest security updates. The new thing with KNOX is that your phone will sit at the boot logo if you flashed to downgrade.
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Also, I noticed you didn't say you cleared/wipe cache/ dalvik and factory reset in recovery. I am assuming you did, but you didn't list those steps?
What is the longest time you let it sit at the logo?
What model number does your phone have?
The flash you attempted, was it lower or higher than what was on the phone already? You may have to flash it with the latest Android, then side load the latest security updates. The new thing with KNOX is that your phone will sit at the boot logo if you flashed to downgrade.