A couple things you might want to look at that could be potentially easy fixes.
First: Is the lifter rod secured tightly? Make sure it is not wobbling around in the clamps that hold it in place.
Second: Is the action installed correctly? Sometimes if an upright action is installed slightly incorrectly the piano may appear to play almost OK but one end will be slightly farther away from the strings than the other.
However, since you mention that it's only lifting late in the middle of the piano, it may just be damper timing needs to be addressed. Is this happening with the sustain pedal only or is it something that they notice while playing individual notes?
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Geoff Sykes, RPT
Los Angeles CA
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