This is water trapped behind the membrane. This will show when the sun hits it and the water under the coating turns to steam and you get bubbles. This is why we stress that the sub straight has to be dry 10% or less moisture content.
You can get a cheep moisture meter to test the moisture content of the concrete.
Solution: Cut out bubbles. Check the moisture content. Wipe down with a generous does of acetone. This will clean and help draw out the water after it evaporates and reapply coating.
A simple test take an 18-inch by 18-inch clear plastic sheet and put it over the concrete and seal it on all four sides. After 16 hours, if any moisture or darkening of the concrete surface is found, the concrete is too wet for coating. Make sure the sheet does not come in contact with direct sunlight. This need to me done in multiple locations. Make sure to do test it at the lowest point in the pool.
This is an example of new bubbles that has water trapped and usually shows when the sun hits it.
This is an old bubble that has been expanding and contracting over time stretched the coating eventually splitting open.
There are many ways for water to get trapped behind the membrane.
leaky pool due to a cracks in the concrete, leaks around any penetrations in concrete such as around the drain or leaky skimmer.
leaky supply or return pipe.
Irrigation lines that do not have to be extremely close to the pool water can travel.
Poor pool construction. Not providing proper drainage on the outside of the pool. Like a concrete basement if you do not have proper drainage water will leak in and you will have small bubbles.
Ground water, If you have a high water table.
Cracks in the patio around the pool. Washing and rain water can get behind the concrete walls of the pool.
Here are some links to articles about bubbling or blistering.
`