Shares in Provident Financial plummeted by up to 19% early on Wednesday, after it warned a shake-up of its doorstep lending division would dent profits much more than previously forecast.
The sub-prime lender is replacing self-employed debt collection agents with ones employed by the company.
However, it has not had enough applications from existing agents and it has about 450 vacancies.
As a result, debt collection has been "weaker" and sales have fallen.
"The business has experienced higher operational disruption than planned due to reduced agent effectiveness through the period of transition," Provident Financial said in a statement.
Recent vacancy levels have been 12%, it added, more than double the rate anticipated.
"We didn't get it right. The incentives we had in place and the other management actions and communications that were there, were not sufficient to retain the number of agents that we anticipated," chief executive Peter Crook told analysts during a conference call with analysts.
In April, the company said it expected the shortfall in contributions to profits, mainly because of weaker debt collection, to be about £15m in the first six months of the year.
However, recent collections performance has deteriorated, particularly in May, and so the shortfall is now expected to be up to £40m.
Sales to existing customers and customer retention had also been hit, and so credit issued in the first five months of 2017 was £37m lower than last year.
Provident Financial said debt collections were "stabilising" in June because most of the new doorstep collection jobs had now been filled, and would begin to "normalise" from July onwards.
The disruption to the consumer credit division is likely to see pre-exceptional profits fall to about £60m this year, compared with £115m in 2016, the company predicted.
FTSE 100 member Provident Financial also owns Vanquis Bank, and consumer credit brand Satsuma Loans. It also owns Moneybarn, which specialises in sub-prime car loans.
The shares are now about 16% lower.