time machine not backing up ...says 'in progress but never finishes

That's one reason it's a good idea to have more than just one backup drive. If one keeps working while the other one stops for whatever reason, it effectively isolates the problem to that drive. Reformatting it often fixes the problem, but it's never a permanent fix. Besides, you don't want to be without a backup even for a short period of time.

Backups are like engines. Just one engine... really isn't enough.

I understand TM stopped backing up for no apparent reason. For fundamental troubleshooting references, please read If you can't back up or restore your Mac using Time Machine - Apple Support and Time Machine troubleshooting - Apple Support.

... i go to the time machine backups folder and it shows the time and date and indicates 'inProgress' but never finishes

Don't bother opening that folder. You can look at its contents if you wish but no actionable information exists within.

Try "reconnecting" to the backup disk as explained below.

The following technique can be used to "reconnect" to the backup disk. It will not affect the existing backup history in any way.

Open Time Machine's Preferences and de-select the Back Up Automatically checkbox. Make sure no backups are in progress, otherwise you should wait for it to finish or terminate.

If the backup disk is directly connected to the Mac, disconnect it: drag its icon to the Trash, wait for it to disappear from the Desktop, and then physically disconnect it from the Mac.

• If its icon does not appear on the Desktop, select External Disks in the Finder menu > Preferences... > General. You can de-select it later.

Then, click the "Select Disk..." button, select the backup disk, then Remove Disk, then Stop Using This Disk.

• If you designated more than one backup destination, the "Select Disk..." button is replaced by an "Add or Remove Backup Disk..." option. You have to scroll through the list of backup disks for that option to become visible.

This will not affect any existing backups.

Reconnect the backup disk to the Mac, power it on if required, and wait for its icon to appear on the Desktop.

Then, Select Disk... again in Time Machine's Preferences. Re-select the same one under Available Disks, then Use Disk. If the backup is on a network (TC or AEBS), you will be prompted for the password you provided for that disk in AirPort Utility.

"Waiting to complete first backup" will appear, which means it won't do anything until the next scheduled backup. Despite the implication of that message, it does not mean it needs to create a completely new backup, as if it's creating a brand new one. Your existing backups will still be available.

If you don't want to wait that long, select Back Up Now.

A Notification may appear if your source volume is encrypted and the backup volume is not. That's normal.

Re-select Back Up Automatically. The "Preparing Backup..." status message will remain for a long time. Eventually, "Backing up xxx of yyy" will appear, but those values will not be accurate, nor will the "Estimated time remaining" in Time Machine's Preferences. Just ignore it. It may take a few hours even if your Mac is not allowed to sleep. Time Machine will also pause or become slow if you use your Mac for tasks it considers a higher priority.

Needless to say, if you are using, or have used at any time in the past, any non-Apple "anti-virus", "cleaning", or "Internet security" products including "banking security", anything at all in that broad category of utterly useless garbage, don't. A lot of things won't work if you do.