Word for "getting careless and slacking"

Solution 1:

Consider:

complacent :
marked by self-satisfaction especially when accompanied by unawareness of actual dangers or deficiencies
: marked by complacency

• The strong economy has made people complacent.
• We have grown too complacent over the years.
• We can't afford to be complacent about illiteracy.

The person in your question would have become complacent in their job.
When Susan first started, she did her job admirably, but she has recently become complacent and it's starting to show.

Solution 2:

Consider coasting

to continue without serious effort, letting one's past efforts carry one along

There is also an idiom phoning it in

Perform an act in a perfunctory, uncommitted fashion, as if it didn't matter.

Solution 3:

Please consider:

LACKADAISICAL adjective : feeling or showing a lack of interest or enthusiasm: lacking life, spirit, or zest : languid

from m-w.com

Solution 4:

If you are looking for a noun, slacker might fit the bill:

A person who avoids work or effort.