What to do if referees take a long time to upload their recommendation letters?
My experience comes with asking for recommendation letters with my PhD application, most of the times it was necessary to remind the referees about the deadline for submission; they're so busy that they easily forget about it.

Another option would be with resending notifications through the recommendation system; many of such systems allow for sending automated reminders to the referees thus eliminating the need for direct intervention with the busy professors!