Ok, I'll summarize my previous reply for both of you:
- try the Vuforia Image Targets sample;
- if the sample works, the issue is with your application code
- in that case, you can do 2 things:
1. compare your own application code with the one in the sample, to see if you have forgot to do something, or if you do something different with respect to how to load datasets, how to activate, how to attach the augmentation, and so on.
2. read the developer guide
On the other hand, if the Image Targets sample also does not work, then it could be a device specific issue (in which case I need the details of your device, such as Model number and Android OS version installed on your device).