One slip can be a small job
If the surrounding roof is secure, a single slate can often be re-fixed or replaced locally.
Repeated slips suggest the fixings deserve attention
Hooks and tabs from earlier repairs are common. If new slips keep appearing in different places, inspect the broader fixing condition.
Thickness and size matter
A replacement that is too thick lifts the course above. One that is too short loses the overlap that keeps wind-driven rain out.