I have done an intense shock and the pool was blue, clear, and free of algae. This would last for several days (up to almost a week) then it would return, which I think happens when the shock levels drop and due to the possibility of the cell not working properly, the chlorine level is not maintained. I went through this process over and over again. When it was shocked and chlorine was manually added, it was then clear and there was no algae at all. The problem is that it then returns, and the vicious cycle starts all over again.
It may have been clear but if you did not do an
Overnight Chlorine Loss Test you cannot be sure it was actually algae free.
The
SLAM Process is not a once or twice addition of chlorine.
It is raising fc to slam level for your cya & MAINTAINING it there until ALL 3 of these are true:

CC is 0.5 or lower;

You pass an
Overnight Chlorine Loss Test
AND

The water is clear.
(Crystal Clear w/no algae dead or alive)
After which you can immediately do an Overnight chlorine gain test to determine if your swcg is working properly.
If you wait until you have inadequate fc for your cya level you will need to start all over again as algae will again take hold. You must continue to manually chlorinate until you get the cell problem resolved.
It does sound like you may have a cell production problem but we need to be able to prove it - & you will too if you want to put in a warranty claim. This means taking care of the algae first.
We suggest liquid chlorine for the
SLAM Process & daily chlorination instead of trichlor, dichlor, or cal hypo as it doesn’t add cya or calcium which can cause issues if they are allowed to get too high.

As we mentioned you must have
your own adequate testing supplies to complete all these procedures & know where you’re starting from. Without them you will just be blindly throwing money into the pool & hoping it works out.
You must have at minimum:
- an fas/dpd test
(Accurately tests fc & cc separately from 0.5 up to 50ppm), imperative for maintaining slam level & performing oclt & ocgt
- cya turbidity test
(So you can be certain of your cya level & chlorinate accordingly) otherwise you may be using too much chlorine which is wasteful & can damage surfaces/equipment/swimmers
Or using too little chlorine which will be ineffective in your fight against algae.
You also need to know all the other parameters- ph, ta, ch & salt (measured independently of the swcg readout)
Test Strips don’t cut it.
The Taylor k2006C + k1766 salt or tf100 salt have all these.
Without accurate data we are just guessing.
Please check the links that kim posted about sourcing the proper tests.
***Until you can get a kit in & test yourself you can add
5ppm worth of liquid chlorine (sodium hypochlorite/ plain bleach)
to the pool each day infront of a running return to keep things from getting worse. NOTHING ELSE.
Use
PoolMath to calculate amounts using your pool’s volume & bleach strength %.