I’d been retweeting using Hootsuite for a while, but, one day, the ‘Retweet’ option was disabled (grayed out).
I had a little trouble finding a solution, but this is it. Go to Settings -> Preferences
Make sure that “Use Twitter Web retweets” is checked…
…and voilà, you can now retweet as before:
Solution source: https://forum.hootsuite.com/discussion/6614925/retweet-button-disabled