Some of the most common challenges for me have been late payments, vague project scopes (clients not putting what they need in clear enough terms), and scope creep top the list. Additional ones I can think of include unrealistic deadlines, inconsistent communication, and the constant hustle to find new clients. These small but frequent irritations have sometimes turned my career into a juggling act of patience and persistence.
However, you have to hang in there and earn as you go. You can learn to notice these things more easily as you become more immersed in this world.