YOU SAID:

He is bad a chess, he does well sometimes

INTO JAPANESE

彼はチェスが下手だが、時々上手にできることがある

BACK INTO ENGLISH

He is bad at chess, but he can be good at it sometimes.

INTO JAPANESE

彼はチェスが下手ですが、上手になることもあります。

BACK INTO ENGLISH

He is bad at chess, but he can be good at it.

INTO JAPANESE

彼はチェスが下手だが、上手になることもできる。

BACK INTO ENGLISH

He is bad at chess, but he can become good at it.

INTO JAPANESE

彼はチェスが下手だが、上手になることはできる。

BACK INTO ENGLISH

He is bad at chess, but he can become good at it.