trunk doesn't auto open
 

07 a8, the trunk wont auto open (but it auto closes) after I press the button, it releases but after 3-5 sec it locks down again. It opens only if I give it a little help from the beginning, then it does open all the way up, suggestions guys? the motor is working though

Closing is always the easiest for the motor...opening depends on spring tension and freedom of movement for the final clearance of the latch. I would check the trunk hinges for binding or need of lubrication. Also, the trunk seal...esp if cold weather is a factor. Coating all door and trunk sealing rubber with silicone spray is always a great idea, esp in winter...even improves the air/water tight seal, while preventing it from sticking.

If the lube and tune doesn't work, visually check the hinges for any misalignment. Its common for those to get slightly bent in cold weather if the trunk binds to the seal or if its trying to lift a butt load of snow.
If the trunk hinges look good, reach up above the battery and you can find the motor. (its big) remove the bolts by the trunk hinge bolts and pull it out of there, if you find any cracks in the case you need a new motor.