Can't rule out pushrod flex as a contributor, but even good roller tappets can't take any air gaps - the needles just snap and escape, the roller wheel collapses onto the axle, blah.
Even though this may not be a quality problem at all, you must examine the other 15.
The Harley-Davidson roller test (since they all have them since 1929) is:
1. does it rotate freely, no scratchy spots or drag
2. end-play is the same on all tappets
3. zero up-&-down play