Here's why you should have a learning curve timeframe when you train staff for mission critical skills. 
First, understand that each person learns at a different pace. Second, provide the necessary tools or resources for training. Third, allow for eventualities in personal circumstances.
After factoring all the above into your time frame, communicate the timeline to your staff. Send reminders. Ask questions to ascertain if there are allowances to be made.
If your staff still do not learn or show an improvement in skills, then one of two things must be true: they are incapable of learning or they are unwilling to learn.
In view of these two facts, make a financial and executive decision.
