Or more precisely, consistent smash, inconsistent clear. Some days my clears are effortless and powerful and other days it's a real struggle to get from baseline to baseline. Sometimes I'll have a month of good clears where I think the problem is solved... and then I'll revert to struggling. The problem isn't movement or positioning. It might be contact point. It might be timing. On the days when I'm clearing well I notice my arm is relaxed and I get a good strong "whipping" motion before contact. However when I try to emulate this on the bad days rarely do I get results. The strange thing is my smashes are powerful, accurate and consistent. If my smash is good why is my clear such a mess? Will try and get a video but maybe somebody else has experience this problem?
Without the video it is only guessing, but this... ... sounds like you might already have your answer. You're not relaxed. That's a mental thing and trying to force the relaxed motion won't help. That's also the only explanation I can come up with for the smash not being effected. That only makes sense if it's in your head. Now... giving advice is quite hard, if that's the case. Maybe don't try to force it, instead accept the bad day for clears and focus on other thing. By getting the focus away from the clear you can at least work on something else - or it might even help your clear on that day. And now I'm waiting for your video of a good and a bad day...
When you say he is "not relaxed." I suppose you mean just when he is doing the clears. Not that he is "not relaxed" period. E.g. not to say that he is not relaxed for other shots like when doing the smashes. Though funnily enough I have seen some very stiff players that play ok strength smashes but not such length or height in their clears. So I don't know if maybe smashes can be more forgiving of stiffness in some way! Though of course power is lost from stiffness.
Hit the shuttle at the highest points, keep the arms straights (like pro basketball dunking), Hit it forward instead of upwards, apply scissor steps if needed. Sent from my POCOPHONE F1 using Tapatalk