Police in West Yorkshire have secured only two convictions for people trafficking offences since 2005.
Information supplied by the force reveals in the years since then, there have been 21 human trafficking investigations, with six taking place last year.
There were three in 2012 and five in 2006 but none were recorded in 2008 or 2009.
Six people have been charged as a result of those investigations but only two have led to convictions, according to figures released by the force.
West Yorkshire Police is also working to provide improved training on people trafficking for staff and so far 250 people have completed a three-hour training package which is aimed at investigators and front counter staff.
In addition, 5,700 police employees have undertaken an e-learning training package on the issue, which takes about an hour to complete.