Bookmaking is a rough business.
You go in knowing long term you WILL be profitable - but collecting paper profits is not the same as winning on paper.
You get to rough in collecting - you invite outside interference (ie law enforcement) - you go soft - word gets out and the deadbeats will over run you. The "middle ground" - using outside assistance in collecting your debts - of course these type of guys with the muscle to collect will eventually realize you need them more than they need you and eventually you work for them.
Then there's the capitalization issue - again - long term you will win - but smart bookies never lay off the action. Again - bet with other guys and lose - your giving away profits, bet with them and win - no guarantee they don't stiff you and leave you in the same tough spot.
Volume is the key - both in keeping players active constantly to churn out a profit and in keeping their debts (and payouts) on smaller side so they can be managed. Once someone gets in too deep - they have no incentive to pay. They know they are buried and move on to greener pastures to play. Always some sucker who will extend credit to a new customer in hopes of making $$.
Finally - you do too well - imitators will rush in to steal your clients. Again - always some dumbass who will over extend to losers and steal your most valuable clients. They then lose big to this guy - and best case they return back to you chasing - but maybe they check out of the scene altogether.