What can cause headset bearings to become loose quickly? On a new-ish CX bike (about 1 year old), I frequently need to adjust/tighten the headset bearings (semi-integrated ahead type). 

After a few rides, they become loose which is noticeable by a "shuddering" feeling when using the front disk brake. Additionally, there is noticeable play in the bearings when I pull the brakes and move the bike back and forward. 

What can cause the headset to become loose that quickly? What can I do about it? I already tightened all screws on the stem to spec (the label says "all max 8Nm") after tightening the adjustment screw, is there anything else I can try without specialized tools or replacing the headset?
A few months ago, I finally found the most likely reason for my problem: Instead of a star nut, this bike uses an expander plug under the top cap. This plug extends about 4mm above the end of the steerer tube, like this:

######### 
|#######|
|#######|
|#######|


Where the # represent the plug, and the | is the steerer tube. I had the stem at the topmost position, and I suspect that it not only gripped the tube, but also the compression plug. Which in turn was loosened by vibration and the stem moved upwards with it, loosening the bearings.

I have since moved a spacer above the stem so that it does not touch the plug; this seems to have fixed the problem. The lower handlebar position is fine with me, and my problem hat not re-occurred for the last months despite riding some rough-ish trails.