可是， 然而，不过，但，然，可 are similar to 但是in meaning and usage, 但是/但（but）stresses the relationship heavily than 可是/然而/不过/然/可（however）。

可是 express twist, 但是 has progressive meanings

Hello,nice to meet you. ”但是“ just said turning，but”可是“with the emotional color that you not to wish.

Perhaps you could give a few examples of where native Chinese speakers change your "可是" into "但是"? That will give us a sense of why they are correcting your usage of the expression. I'm curious because both are almost interchangeable, so I would also like to see why they've been making those suggestions.

你还可以说“不过”。 eg：我饿了，不过我还不想吃。（最轻的语气） 我饿了，可是/可我还不想吃。（中等语气） 我饿了，但是/但我还不想吃。（最重的语气） 一般来说，“但是”用在口语和书面语上，“可是”只用在口语上。