Jagdflieger unconfirmed victories and official confirmation of claims

[Nikita Egorov]

Scanning through some of records I found out that the majority of German fighter aces have lots of unconfirmed victores on the their accounts, e.g. Lipfert - 27, Johannes Wiese - 75 (fantastic!). My question is what could be the ground, exept lack of witness, for declining claim during official procedure?
Second question is that as far as I know Hartmann and Barkhorn have no unconfirmed victories (please correct if I am wrong). Does it mean that every claim they filed was confirmed by headquarters and what was the reason for that taking into consideration the fact that in some occasions (ecpecially with Hartmann) they even failed to identify the exact plane type they shot down (unsubstantiated "Lagg" are recorded)?
Third question is that how deep personal relationship could effect the claims confirmation procedure in Jagdwaffe? I mean cases with well-known "neck itching" decease. For example Hermann Graf who claimed fantastic ammount of victories during Stalingrad battle and whose 200th claim is not confirmed by Soviet losses as it is in "Graf-Grislawski" book...

TIA