Not much information to go on to give you much help.
You posted in the mechanical slot section. We can assume you have an IGT mechanical slot machine. Usually the coin comparator is mechanical in those machines.
I would suggest -
Post pictures of your machine. One of the front, one of the inside, one of the door open with the coin comparator.
I'm sure if you search the forum you posted in, you can find some similar posts that may help with your problem, or at least get you to the point you can ask a bit more specific question after some looking and trying some of the things other members have done to correct that problem.
You can also search the coin comparator section and see if anything posted there might give you an idea where to start to get your coin comparator working.
There are many members here that can help and would be more than happy to do that. Pictures are actually worth a thousand words when it comes to helping here in the slot forum.
If you have trouble posting pictures, there are many posts on how to get then uploaded. They must be less than 1G (1000Meg) in size and you can post about 5 pictures per post as long as they are all less than 5G in size combined. You can probably most more than 5 pictures if they are are under that limit.
Let us know how it goes and I'm sure the members here can help get your machine working in a short period of time.