They might not taste as good as they did when I first bought them.
この場合、次のような言い方ができます。
ーThey may have lost some of their flavour.
「味が落ちているかも」
to lose some of their flavour で「味が落ちる」ことを表現できます。
ーThey might not taste as good as they did when I first bought them.
「買った時より味が落ちてるかも」
not taste as good as ... で「…と同じくらい美味しくない」を使っても「味が落ちる」を表現できます。
ご参考まで！
カナダ 
