Well, these could be one of the reasons -
1. Check again, your Sim might be doing another task. Make sure he/she isn't. [b][/b]
2.I figured this one out. Out of 7 Sims, I sent 6 of them to work. After they did that, I tried to send them to work again, but I couldn't see the flashing suitcase ! Then when I tried to send the 7th Sim to work .... IT WORKED !!! I'm serious.
REASON : SIMS IN THE SIMS FREEPLAY CANNOT GO TO WORK MORE THAN ONCE IN A DAY. TRY AGAIN THE NEXT DAY, I PROMISE IT WILL WORK. PLANT SOME PLANTS INSTEAD.
If this helped you, please let me know !!!