migration from old iMac

How are you trying to migrate?

Using your local Wi-Fi network is to slow and unreliable, especially if you have moderate to large Music, Photo and Doc. libraries.

Using an Ethernet to Ethernet connection between the Mac’s is slow and only slightly more reliable.

The fastest, easiest and most reliable way to migrate, is to connect your old Mac’s Time Machine backup drive to the new Mac and migrate from it.

If you do not have a Time Machine backup of the old Mac? Then you need to get an external drive and make one.

see > Back up your Mac with Time Machine - Apple Support

Plus then you can do the right thing and backup the new Mac with that external drive.

What are your issues? Are you following this process:

Move your content to a new Mac - Apple Support

Please follow the steps in Setup Assistant and Migration Assistant. Do not use any other method unless you want to waste time and risk a failed migration.