carnation going downhill

[Marb67]

Well here I go again, another shop bought plant going downhill as soon as it's over my threshold. I purchased this healthy pink carnation plant with lots of flowers a few weeks ago. In windowsill in the kitchen but over time flower buds are getting smaller, going brown before opening and leaves also browning. For the life of me I don't know what has happened to make it go like this

[roitelet]

Possibly over watered, they thrive in dry soil. If it’s hardy it would be better in the ground.