The first possibility is unlikely but worth checking. You should ensure that the lower lobes of the MRS hub are not cracked on the rising blade.
The next quite likely possibility is to ensure that you do not have cracked main rotor blade tabs. Use a magnifying glass.
There is also a replaceable spherical bearing inside the Blade cuff on a beam that attaches the flex beam to the frequency adaptors. I would check that as well as it can turn into a maraca quite swiftly once it starts wearing, this will also affect the track and hence vertical balance. It is best practice to change all of them if you find one worn.
This depends on the blade part number. The blades do have a tendency to crack and craze a bit along the trailing edges but this should not affect the flight of the blade. The crack to look out for is one that starts about a metre from the cuff on the trailing edge and propagates at an angle of 45 degrees. But I have seen them up to a couple of inches long and only found them on a detailed inspection so they don't appear to affect the flight of the blade. That one will cost you a few bob unless you have been wise enough to buy PBH.
Last but not least: It is always best to raise a Technical Enquiry (TE) in your Keycopter account. If you haven't got one then please do apply. It is worth it. You will have access to the horses mouth and your snag will then become part of product support and improvement for everyone.
I hope this post finds me blathering into the ethernet (which I rarely do) and your engineers have already fixed the problem.