Cannot level up some ninjas in li<x>nk Assist?

On 2019-01-04 22:26:51 ｜ Show this Author Only ｜ Descending Order

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I have a question about link Assist.

I got Itachi Susanoo last month as well as Chiriku, and I was wondering, why these two, among multiple more ninjas, are not able to be upgraded/leveled up in link Assist for power bonus in the Enhancement tab?
For such ninjas there is a special stamp that writes "Enhancement". What it even supposed to mean? It doesn't really tell you whether you can or cannot, neither indicates whether or not it will be in future or not... what it means?

Sasuke Susanoo can be, 4th Raikage can be, Hokage Minato and Kurama mode Naruto can be as well, Edo Hashirama and Tobirama with Hiruzen can be (I don't have the ninjas that are mentioned after 4th Raikage, but they show up as - Not Obtained obviously, but doesn't have "enhancement", indicating that they can be enhanced/leveled up).
Meanwhile GNW Ten Ten cannot be leveled up, three versions of Termari, both 1st and 2nd Hokages (non Edo), Angel Konan, Itachi Susanoo cannot be, and such as Omoi, show up with a red stamp writing "Enhancement" even if I have nor don't have some of the mentioned, and they cannot be leveled up, specifically the ones I have. But those that I don't have, have the same stamp written on.

Can a moderators, or staff members, or maybe a player who somehow knows the legit official answer, explain this?
Here are the examples.
NarutoOnline190104161824
NarutoOnline190104161853