You're likely trying to delete /Library/Preferences/com.apple.dock.plist You want to delete the one on your home directory (/Users/CapU/Library/Preferences/com.apple.dock.plist). If you have multiple users, you'll want to loop through them. -t-

it depends on how you ran this command:

defaults delete com.apple.dock

If that was run directly in Terminal logged into your account, that would delete the Dock.plist for your account and should have reset the Dock.
If that was run from a policy, that's going to try to delete the Dock.plist from the root account, which isn't going to have any effect on your Dock.

If you did the former and not the latter, try throwing in a killall cfprefsd after the deletion, but before restarting the Dock. It may have just cached the previous Dock information and reloaded it.

I've only ever used defaults delete to delete keys within a plist file. If you want to completely reset it, you'll probably need to flush the preference cache first, then use rm on the file, then finally killall Dock.

Hmm, good point @davidacland! You probably should rm the file and not use defaults delete here. Wasn't really paying attention when I read the post.

Thanks for the respopnse.
How do I "loop thru" again?
I know dumb question

Basic idea:

#!/bin/sh

for a in $(ls /Users/); do
    rm /Users/$a/Library/Preferences/com.apple.dock.plist 2>/dev/null
done

You can set it as a policy to run at login for each user on each computer (depending where you need to reset it), using $3 as a variable for the username, i.e. /Users/$3/Library/...

I tested

!/bin/sh

for a in $(ls /Users/); do rm /Users/$a/Library/Preferences/com.apple.dock.plist 2>/dev/null
killall Dock
done

The dock never refreshed to the default state.

As @davidacland already mentioned, you likely also need to do a killall cfprefsd in there or it will simply load from cache, not from disk. Its just "one of those things" we need to do to get around Apple's aggressive caching model.
If that isn't working, remove the 2>/dev/null from the rm line and run it again and see if it shows any errors. It might be balking at something.

Also, are you certain you don't have a Configuration Profile controlling the Dock in play here? Please double check that, since no amount of deleting plists is going to help if your Dock is being managed by an immutable Dock config.