ADT eclipse插件安装 为什么总是出现以下情况，而安装失败

Cannot complete the install because one or more required items could not be found.
Software being installed: Android Development Tools 12.0.0.v201106281929-138431 (com.android.ide.eclipse.adt.feature.group 12.0.0.v201106281929-138431)
Missing requirement: Android Development Tools 12.0.0.v201106281929-138431 (com.android.ide.eclipse.adt.feature.group 12.0.0.v201106281929-138431) requires 'org.eclipse.gef 0.0.0' but it could not be found