An individual aircraft can lose its ETOPS certification for a number of reasons, a component change for example, with some systems being considered more critical than others, fuel indication for example. The first half hour of a flight can sometimes be used to revalidate the rating ie if it works OK during that time its considered fixed, if it doesnt you turn round.
Another reason may be a defect that is being carried until decent downtime allows it to be fixed, say for tank entry, this could limit its ETOPS ability or nulify it completely.
An operator may choose some aircraft to be none ETOPS within the fleet for cost reasons.

Richard