i cannot upgrade my nexus 7 2013 from 4.4,2. to 4.4.4

[AC Question]

i have just had my nexus 7 (2nd gen) repaired, USB port replaced. They factory reset it to 4.4.2. now it won't upgrade to the suggested 4.4.4. I have cleared the cache. On finishing upgrading it says it's finished but does not boot up and stays on the same screen. On checking it still shows 4.4.2. Advice appreciated, thanks.