There are several things that can be done to try and solve this issue.

1. Make sure that you have updated to the latest version of Lemmings.
2. Ensure you have Wifi and/or data and are connected.
3. Try restarting the app completely (full shutdown and restart).
4. Try restarting your device.
5. Try turning data and/or wifi off and then on again.
If you are still having issues with performance, there is the option of clearing the App Cache data which could help. 

In order to do this:
1. Go to the Settings menu on your device.
2. Tap Storage. Tap "Storage" in your Android's settings. ...
3. Tap Internal Storage under Device Storage. Tap "Internal storage." ...
4. Tap Cached data. Tap "Cached data." ...
5. Tap OK when a dialog box appears asking if you're sure you want to clear all app cache.