Know thy enemy
What you are up against, is most likely a bildup of lime, PLUS fatty residue that splashes on the glass when you shower, enhanced in stubbornness by additional compounds from your soap or shampoo. (Perhaps the bildup started when changing products, e. g. Started using an anti-dandruff shampoo that contains selenium silicate )
The end result is that the lime ends up encapsulated and mixed with fatty compounds and that makes it harder for acids to dissolve as they normally do.
Acids may seem like a good solution
Someone suggested phosphoric acid that is probably the worse idea since phosphoric acid is the only acid that attacks glass.
The pumice stone up there is a good idea however it is still an abrasive solution to a problem best handled by solvents.
Now, acids will not work on this residues so, my advice is to head over to the other end of the pH scale.
The other end of the pH scale
What you need is a strong base, Sodium Hydroxide being the most chemically reactive substance that you can get your hands on.
It is sold as an oven cleaner on most supermaekts, under the generic name caustic soda. It is also sold as a rooter for clogged drains since it is specially adept at dissolving hair.
Now days, most of this products come in a diluted form to be applied as atomized spray or aerosol foam, however I would recommend going medieval on your lime and apply full strength with prejudice. So be sure and get the real thing which should be a viscous off-white liquid with the same consistency as egg-yolk. That is pure unadulterated caustic soda.
Back in the day, easy off used to be sold in small wide-rimmed bottle with a little brush for application. That's what will get rid of your soapy fatty lime residue.
Solid granules or powder
If all you can get your hands on is the solid variety (dehydrated or deactivated lye) you will need to dissolve it in water. Make sure you add the lye to water, and not the other way around. Do this on a well ventilated area, as doing this will release caustic fumes and also heat (it's an exothermic reaction ) here is a video showing bhow to do it properly. Notice the consistency of the solution and the safety gear used. Also, this is done out in the open, since an extraction hood was not available to eliminate the fumes.
Glass is impervious to Sodium Hidroxyde, however in the reaction with lime, after dissolving the encapsulated fat and soap will vaporize and create chemically reactive fumes which if accidentally inhaled will at best irritate and at worse cause chemical burns on any mucous membrane they come in contact with, (eyes, lips, sinuses) so it goes without saying, WEAR A MASK AND GOGGLES in addition to rubber gloves when handling caustic soda. A handkerchief and eyeglasses will do, but ventilate the area appropriately after application.
Also, water will make it "jump" (if you drop some on the toilet, for example)
As for the cleaning procedure, just apply a coat with a brush, let it sit for a few minutes then clean up with a wet disposable fabric towel. Then rinse thoroughly with running water. Do not use the shower head for that purpose since that will make any caustic soda film that you may have missed jump in tiny droplets, that will probably cause tiny skin burns or may dissolve and create tiny holes in your clothing.
Caustic soda is probably second only to nitric acid in chemical reactiveness and deserves to be treated with respect.
if all else fails
If that doesn't get rid of the residue, there is only one other solvent that may get rid of it, but I don't recommend it being handled by anyone without laboratory experience: Piranha Solution Wich is a reactive mix of Sulfuric acid with Hydrogen Peroxide. and is used to clean laboratory glassware from stubborn organic char residue.
Try your luck with Sodium Hydroxide first