In a very rare occasions server returns ERROR_NO_SLOT_AVAILABLE which means the queue of captchas that are not distributed to workers is too long. Mostly that happen with ReCaptcha V2.
Customers are asking to implement priority/premium access system for those who ready to pay higher price for priority over other customers so they never face this error. But we think that this change will affect service rather in a bad way and that's why: yes we may earn more but we will have less motivation to improve our service.
This is how it works. To solve more captchas we need more workers online. The more workers online, the less each worker have captchas to solve. So workers have to wait longer to get new task. The longer workers wait for new tasks the less workers online. Therefor the only way to have more workers online is to give them more tasks to solve.
In a day we will update you on how we gonna produce more tasks for workers and therefor improve the workers' online pool to reduce ERROR_NO_SLOT_AVAILABLE issues.