Greetings. I recently had a piano that had one key in the 6th octave that is that is blocking against the front keyslip. This is on a new Essex small grand. I made a few adjustments but ended up just putting a piece of cardboard as a shim (between the front of keyframe and keyslip)...this was a temporary fix that hasn't lasted more than a couple days.
Any ideas for a more successful correction?
Thanks.

I suspect this piano is under warranty, so I would probably contact Steinway for advice. There are ways of dealing with things like this, but I would not like to do something that would jeopardize the warranty.

You can try with the capstan screws bored into front side of the key frame, between it and the keyslip, if there is enough space there. You can regulate the size of the gap between them by adjusting capstans. Some factories have that on their pianos.

If it is really just one key, you may be able to move the key back by bumping the balance rail pin back. Of course this only works if the pin is not already back, and you must be careful that the balance rail pin doesn't bind when you depress the key.

I could see that on an old piano (Steinway)
whereas the keyslip warps over the decades but on a NEWER piano whereas they usually have an aluminum rail or ?, warpage is not usually the culprit. There is something else going on with that one key. Actually I like that capstan idea.It probably looks like a professional fix. Usually we have to make a new keylip on really old vintage pianos. Hassle!