The suction cup ones never worked for us. Always fell off immediately. We got Moen HomeCare (but a different style than the ones Mitchell installed).
The suction cup models work well on CLEAN glass, or CLEAN SMOOTH tile. If your tile isn't smooth/glossy or isn't clean, the vacuum seal is compromised and as you note it won't work. I have a pair of them here at home that I used for my Dad (who is now 100) when he came to visit and prior to me installing the permanent ones. We have smooth tile. Wet the cups, and they work like a charm if you renew the seal regularly.
Yesterday, I installed THREE more safety bars, kind of an emergency. Got a call from a local visiting nurse assigned to a case; an elderly veteran was coming home from the hospital on Wednesday afternoon. The safety bars were essential for him to use the shower safely. When I arrived they only had ONE vacuum mount one, vertical on the glass to assist in exiting the shower.
The ones I installed yesterday were all identical; peened stainless steel, concealed fasteners. All three were 18" length.
All three went into studs. If you look at the photo with two bars, the one on the right is to assist the patient in standing up from the seat in the shower. The one on the left is to assist the patient in getting to the shower exit. The vertical one is to hold onto as he exits the shower threshold.
There's a system called "Wing Its" for installing these in walls when you cannot find the studs. Unfortunately, the system requires a ¾" hole in the wall which is a bit invasive for my tastes, and the WingIts are also $14.00 a pair. Hillman fasteners makes a toggle bolt that only requires a ½" hole; it's a threaded solid rod of zinc that takes a ¼-20 screw, and it's attached to a zip tie with a small grommet. I've used these too behind tile when needed.