It's been a while since we talked last, but I'd like to ask you a question if I can.
We haven't talked for a while, but I've been wanting to ask you about ...
この場合の「が」に but を使って言えますよ。
ーIt's been a while since we talked last, but I'd like to ask you a question if I can.
「前回話してからしばらく経ちますが、質問したいことがあります」
ーWe haven't talked for a while, but I've been wanting to ask you about ...
「しばらく話をしていませんでしたが、…について聞きたいことがあります」
ご参考まで!