Pool Algae hard to remove

[GalCres]

Dear all,
I own a fibreglass pool which is chlorine-based. Recently I found green/yellow algae which seems difficult to get rid of.
The first initiative I did is shocking the pool by Granular Chlorine which worked well in same day and all spots gone, next day the algae came back, follow the suggestion from pool shop I poured 1L copper-based algaecide and keep running filters for a few days along with daily brushing however no improvement at all. I was frustrated then did another chlorine shock without success, the algea now seems resistant to any form of chlorine.
The pool shop has no better solution at the moment, anyone could help would be highly appreciated!
Looking forward to seeing your suggestions!

[Teapot]

Hi, the first step is to post your water test results on here for us to look at. These need to be proper tests and not with dip strips.
Most of the time the reason you can't deal with algae is because the cyanuric acic level is to high. This compound is in most packaged chlorine products and build up to a high level.
Don't assume the pool shop know this, many are sadly lacking in knowledge.

[WalterVandervort]

I recommend In The Swim Super Pool Algaecide, this algaecide is high- concentrated. I have readed about pool algaecide to clean my pool and I see it could be use for your pool. I found review it in marinetalk you can read more about it there.