Icons won't stay in dock

Some icons won't stay in my dock when I drag them from the applications folder, but others will.

I have tried the following without success:

-gone to library/preferences and deleted com.apple.dock.plist then done a restart

-gone to library/preferences and deleted com.apple.dock.db then done a restart

-gone to library/preferences and deleted com.apple.DiskUtility.plist then done a restart

-used disc utility to repair permissions

-opened an application from the applications folder then whilst the icon is in the dock used right click.options/keep in dock

-deselected dock synchronisation from mobile me sync

None of these measures work though iPhoto & iTunes are not affected by the problem.

The only workaround I have found is to create an alias of an application and drag this to the dock where it does stay put.

Can anyone suggest anything else?

jbs12345

mac os 10.6.7,

Macmini2,1

Dock

Version: 1.7

Last Modified: 25/03/2011 19:52

Kind: Intel

64-Bit (Intel): Yes

Get Info String: Dock 1.7

Location: /System/Library/CoreServices/Dock.app