Sounds like it is time for a new switch, or the need for a new contact-point on the starter.

I have gotten life out of worn down switch by taking a 2-inch length of old garden hose and slitting it so that it can be opened. I then slid the hose over the button. This worked fine for several years.

However, replacing the old button is not a very difficult or expensive undertaking.

Sometimes its not the switch that is worn-down; it is the copper "button" contact-point on the starter body. Replacing that is a little more complicated a job.