Well, you might be following the wrong steps, you might be pressing a wrong button, holding the power button too long, or both.
Here just follow these simple steps which will have you to follow hard reset Acer tablet:
- First, use the Switch button to lock the screen from the rotation
- After you do this, the message for rotation lock will get displayed on the screen
- Now you have to complete the process and Switch OFF the tablet by pressing the power button
- Once your tablet gets switched off, now press Volume Up and Power button together
- When tablet vibrates then release only the Power button
- Now you can use the Screen Rotation Lock switch to unlock the screen and then simply follow the steps that you want to perform
In second case, if you are getting Android robot laying on its back with triangle ! mark then follow the instruction mentioned here – http://www.androiddata-recovery.com/blog/solved-no-command-error-in-recovery-mode-on-android
I Hope this may help you.
Best of Luck!