There is no rule as to what the reason is. Firstly, the person must be a good fit for the team - a team player, right values, work well with the other members, be self driven, passionate etc. If that is ok, then secondly they've got to be able to do the job! Technical skills must align to the salary they are asking for - if they are expensive resources then they really need to do what they say they can. If they are not so expensive then there is more room for new learning and basic growth. None of this is possible without the right attitude. Successful candidates must have both.