The question is - where do you set your minimum standard required? Not all pilots are equal, even after identical training, anyone who has been in the military training system knows that.
In any outfit you will have a spread of ability, most will be the 'average', some will be above and some below and unless you have a training system and management culture that allows the below average to improve (extra training and incentives) to average and the average to above average then you will always have those guys/girls at the bottom of the pile who just make the standard required.
Your only option if they are not want you want in your firm is to fire them and raise the bar so that your 'below average' is above the minimum standard you require or throw money and extra training at them.
Passing a check ride every 6 months/year, even if that is an exhaustive check of exercises, does not give progression, especially to the below average guys who probably need mentoring and extra training but that is expensive.
If you are not satisfied with the product then look to the training, both initial and ongoing but you have to accept that not all pilots are equal.