I have found that plastics in a fish tank - those left underwater for long lengths of time, eventually lose their plasticizers - that which makes them flexible. When this happens the suction cups get stiff. The clear plastic replacement ones you can buy in a package do this fairly quickly. When they go opaque white, instead of clear, they are reaching the end of their lives.
Even the fancy green and black ones that come with bubble bars, filter spray bars, heater covers and heaters also eventually loose their flexibility and need to be replaced.
I replace suction cups often. I have never found any that fit certain brands of bubble bars (green), nor have I found replacements for the ones on the heater covers, so I simply do not purchase that brand of bars or cover anymore.