Certain emails not coming through on my icloud

A few weeks ago I emailed my accountant for advice, and after no response after a week I followed up asking what's with the hold up. They then called me to say they'd emailed me twice... I thought they could be saying that to cover my tracks as I'd never received any emails (and yes I've checked my junk folder). But they emailed again and copied my gmail account in, and lo and behold, it didn't arrive to my icloud email but did on my gmail account.

I haven't been too concerned, however it appears the problem has happened again with an entirely different sender. I've just chased someone for payment (my icloud email is my business email so it's vitally important I don't miss anything) and they assure me the details were sent to me 2 weeks ago.

I'm now worried I'm not receiving all emails, which would really mess with my business as I communicate to large numbers of clients and potential clients and could be losing business if I'm not receiving all my emails.

Has anyone else had this issue and can advise what to do?