If your stabiliser is high, your shock dose is wasted. Here is the test to do first and what to change if the reading is past the threshold.
Most shock failures we see in domestic UK hot tubs trace back to cyanuric acid (CYA) creep. Above 50 ppm CYA, free chlorine becomes increasingly bound and unavailable. Shocking at high CYA is like trying to start a fire with damp kindling: the heat is there but it cannot do its job.
CYA binds loosely to free chlorine, releasing it slowly back into solution. At 30 ppm CYA, the binding is light and a shock works almost normally.
At 80 ppm, perhaps half of your free chlorine is bound at any moment. A shock that should take you to 10 ppm of active chlorine actually leaves perhaps 5 ppm working.
At 150 ppm, the binding is so heavy that even a heavy shock dose has minimal sanitising effect. The water can fail health tests despite reading 5 ppm of free chlorine on a strip.
Use a turbidity tube CYA test. Fill the tube with the test reagent and tub water, look down at the black dot, and stop pouring when you can no longer see it. Read the level on the tube.
Strip-based CYA readings are inaccurate. The colour patches do not match well above 50 ppm and bands skip from 30 to 100 directly on most strips.
If CYA is above 50 ppm, do a partial drain (one third of the volume) and refill before shocking. The reset is far more effective than just dosing more shock.
Switch from stabilised tablets (trichlor or dichlor) to unstabilised chlorine for routine top-ups. Calcium hypochlorite or sodium hypochlorite contain no CYA.
If you must use stabilised chlorine, test CYA monthly and refill at the first sign of creep above 30 ppm.
For bromine tubs, CYA is essentially never an issue because bromine does not need stabilisation. The check is still worth doing once a year in case CYA has come in from another source (a shock product, an algaecide, a topup).
Up to a point, yes, but you are spending more on chlorine and not getting your money's worth. Dilution by partial drain is cheaper.
Mains water in the UK has zero CYA. CYA only comes from chemicals you add to the tub.