1) Being Under Capitalized. Do not be disillusioned to think you can make a full time living on a minimal bankroll.
2) Not having enough sports book accounts funded whereby you are not getting the best number available. Betting -1.20 when -1.14 is available is just burning money.
3) Not following proper money management procedures. There is no such thing as a lock in this business. However risking more than 2% of your bankroll on a game will guarantee you will not be around long term.