While studying ばかりだ I found in a grammar book the following example:
日本語の成績が下がるばかりだ。
It was also written that in this sentence ばかりだ cannot be substituted by 一方だ, but searching on the internet I found many sentences with 成績が下がる一方だ. Now, I'm not sure if I didn't understand the difference between ばかりだ and 一方だ, or I read the grammar book incorrectly. Can somebody clarify it to me?