Here’s some things to check:
1. You need two API keys: one for the front-end (‘client’) and one for geocoding (‘server’). The server key is secured by IP address instead of the referring domain. Make sure both have been set up in the Google control panel and in the MapPress settings screen.
2. Make sure your project has the geocoding API enabled in the Google control panel.
3. Check your usage – Google’s free geocoding limits are small, I think around only 2500 locations.
If the above don’t help, feel free to use the contact form to email me a login and I’ll see if I can find the problem.