This is where you may need to seek HR advice as appropriate.  There are a couple of things you need to consider though and that will help ensure that you are dealing with things as you need to.

Consider if the issue conduct or capability related?

Examples:

• Sally is asked to tidy up the store room area.  She refuses.  This is a conduct issue.
• Dawn was supposed to order a particular product for a customer but forgets. This is a capability issue.

Remember the rule to remember and ask:

“Could they do it if their life depended on it?”

A capability issue will require you to look at what further support and training they may require to try and get them to the required standard.  A conduct issue will require you to be very clear as to what behaviour you expect from them and the potential consequences if they fail to meet this.