this is a bit old but may explain part of the problem

80% of the workers who responded live in the U.S. Another 16% live in India, while 4% live in other countries.

increasing pay is usually a good way to get more participants but not sure if it would help in this case. I'm kind of curious now how many people use mturk in Mexico but not sure how you would find that out.

Also when are you posting the hits? Depending on where you are they are likely in a different time zone and that may effect how many people see the hit. For example I live on the East Coast and do work for someone who live on the West Coast and if she didn't email me ahead of time to tell me she was posting work I would miss it because I'm not just casually searching mturk at 2am my time.

@ChrisTurk any other ideas

 

Not really going to be able to overcome this bottleneck. If you can expand to other SA countries you might get more workers from Brazil/Venezuela as MTurk is popular there but otherwise I doubt you're gonna have any luck recruiting off MTurk for this.