My Answer would be the Indian Context...

By external Recruiters, I would presume that to be Recruiters in a Search Firm / Agency.

The following would be the main pain areas

1. Foreclosure of a position - wherein the position flagged off to the external recruiter has been filled through another source viz. job boards, Employee Referrals, Direct Applicants and such closures are not intimated by the Client on a timely basis – Lack of feedback from the Client
2. Similarly…absence of feedback on the resumes submitted against an open position, on a timely basis. Effort put in sourcing the candidate could go waste as delayed / absence of feedback could result in lack of candidate interest and affect recruiter motivation
3. The candidate referred by the Recruiter for an open Client opportunity, is no longer available, as he/she might have got another job offer..thus resulting in a wasted effort from the Recruiter.
4. Replacement Clause..the candidate successfully placed for an open opportunity, leaves within the time-period of free-replacement (normally a replacement guarantee of 3 to 6 months is offered by the Recruiting Agency for all candidates placed by them), thus resulting in refunding the placement fee collected or adjusting it against future receipts.
5. Non-receipt / delay in receipt of Professional Fees for the services rendered..delaying recruiter incentive and affecting Recruiter motivation.

The above constitutes only an illustrative list and is not exhaustive.
I am open for a call to discuss this further.