Your initial reaction might be to dive straight into disciplinary proceedings, particularly if the situation is creating friction among colleagues. While such action may eventually become necessary, you first need to identify the root cause of the problem. For example, it could be that the employee doesn't understand what is expected of them and is too embarrassed to admit it. So, initially at least, raise it with them on an informal basis, but in a private setting.
In some ways it will be easier all round if the employee is hiding the fact that they don't understand something for example, how to use new technology. If that is the case, arrange for some further training, but do not leave it there explain exactly what is expected of them and set a clear deadline for a satisfactory improvement.
If there is no change, you could go down the capability route. Alternatively, the tasks could be excluded from their job role altogether but do bear in mind this decision could be relied on by other employees in the future.
Conversely, when talking to the employee, you may discover that they are deliberately being obstructive. This could be in the hope that the tasks will be taken off them and things will go back to how they were. In this situation, warn them that you will start disciplinary proceedings if there is no immediate improvement in their attitude.
If the misconduct persists, deal with it promptly doing otherwise sends a message that employees can shirk their responsibilities, undermining your entire disciplinary procedure.
You can make it harder for staff to unreasonably refuse to assume new skills and duties as part of their role via your employment contracts. Do this by inserting a flexible job duties clause. It can require staff to take on new duties, which are not in their original job description according to business needs. However, it cannot require them to carry out roles that are beyond their capability or would demean their skills.
When providing training, ask employees to confirm in writing that they have not only attended, but fully understood, its contents. This should avoid any claims to the contrary.