Date: 2021-05-16 08:25
>> The pads I used are flat, not stepped. <<
Flat refers to the face of the pads, with better quality ones being flatter. Pads without a step are usually... non-stepped pads.
Looking at the zhmelody website, they don't show a photo of the back of the membrane pads. What is it? Some synthetic material instead of leather or bladder?
However there is a sort of photo of the back of the membrane pads on their pad set page, and it does look like they have some kind of step. Smaller and more of a taper than the skin pads, but still a step unlike the leather or synthetic pads.
Enough of a step that the comments on stepped pads are relevant, depending on what diameter pads you used in comparison with the keys.
>> The tone holes do not have any obvious damage. <<
Not sure how you checked, but unless they have obvious chips, it's not often possible to tell by just looking at them. Tone hole sometimes have a gradual dip and the front/back sides are by far more common.
>> I will try to re-seat the pads, if it does not work, will change them to stepped pads. <<
If part of the problem is from a combination of the thickness and the way they sit because of the step, then it might be better to use non-stepped pads. Unless the felt itself is thinner on whatever stepped pads you use. See my previous post for an explanation about stepped vs. non-stepped.