Takeoff In Alt-Hold Mode

Hey guys, I need some help,

I’m running Arducopter 4.1.5 (latest stable version).
I have a down facing rangefinder that is set as my primary height sensor and I’m flying without GPS.
I’m trying to takeoff in Alt-Hold and sometimes the drone stays on the ground or dragging but not going up (sometimes it lifts off fine) - This can be seen in Log_1.bin.
I’m attaching a log file with a failed takeoff followed by a successful one.

It looks to me like the EKF estimation is off. I tried to change the covariance of the rangefinder and barometer and it took off but but I’m not sure it’s the only factor (or the right one) - Log_2.bin.


Logs are in a zip file here: https://drive.google.com/file/d/198DJSwOkVE995MRen1JZmYtmmywuX0Hn/view?usp=sharing