It is important to ensure that the base of the HS403A is properly fastened. Take a look at the FAQ at http://www.holosun.ca/index.php/support/faq

Even torque on the screws is the key.

Do you have any hints for attaching the mount?

You need to remove the mount to install the battery or to change from high mount to low profile mount. Use the small Allen wrench to remove all four screws. Set aside the screws. Install the battery or change the mount as needed. Carefully place the mount onto the scope and then reattach the screws so that they are barely hand tight. Now tighten the screws gradually a bit at a time, first one screw, then the one diagonal to it. This is to ensure the screws are evenly tightened. You should hear the screws "squeak" a bit when then are tight. For a new optic, if you have the time, let the scope sit overnight and re-tighten the screws the next day. As the rubber gasket that protects the battery may compress a bit. You can also use a low strength thread locking compound like Locktite Purple(tm).