Provided that you did everything right.

2. Floculant

You can use a floculant. It is also a chemical that will move all small particles to the bottom of the pool. And then, you just pull these particles with a vacuum cleaner. This is a fast -acting way to get rid of algae. Provided that you did everything right. This option will also cost relatively not expensive, but will require effort. First you need to close all the pumps, and make sure that the water does not flow from the filter. After that, add the recommended amount of chemicals to the water and give water to circulate inside the system for about two hours, and then turn off the circulation. Leave everything as it is for the night, giving chemicals the time to transfer particles to the bottom of the pool.  In the morning you set up a filter for pumping water and start to vacuum. Throw the garden hose into the pool while you vacuum. After you carry out the procedure, conduct double chlorination.