I thought of commenting on this thread yesterday, but didnt because I dont own the Axe CP 100.
I do own a Blade mCPX and out of the box it had too much negative and too little positive pitch. There were two ways I could go about correcting that. One was to remove the cyclic servo linkages and adjust. The other is to go into the radio programming and change the travel adjust for Pitch. I chose the latter.
Now, the question is...does the Axe 100 CP use the same type of swash mix...therefor giving you basically the same 2nd choice as I had with my Blade heli. Of course you could adjust the individual links too, but I found that to be too much of a pain with the possibility of breaking a link ball.
I doubt its your motor. If it was...you would be low on power/thrust in either direction, positive or negative. It sounds as if you have plenty of power for positive, but just not enough negative pitch.
Keep in mind that once you mess with the Travel Adjust of the Pitch menu...you may have to go in and reset your pitch curves for Normal, Stnt1, Stnt2 flight modes. Because you always want to be around 0-+1 pitch at mid stick.
I just eyeballed my pitch really as I had no tool to gauge it, so I would make an adjustment and fly the heli and readjust if needed. I realize they do make pitch gauges for these small micro CP helis...I just didnt feel it was worth owning one. I got my mCPX flying just how I like it without one.
Again...I seriously doubt its your motor. A possibility of course, but doubtful...especially going by your description of the symptom.
PS...I forgot to mention. Rafael may be onto something as well. Most heli pilots that have been flying a while will not set up their pitch curves to be 0 pitch at low throttle stick in Normal mode. Most try to attain about a -2 at low stick position. However, in Normal mode you wouldnt have enough throttle at below mid stick on throttle to give any lift. That is the only reason I discounted his suggestion. But, depending on your throttle curve, you may in fact have enough headspeed at below mid stick to provide some lift when flipped inverted. Still, the question is worth asking in order to troubleshoot any and all possibilities that could cause your issue.