When defining elastoplastic anchors, you can define a maximum force in the anchor. However, in PLAXIS 2D 2010 the maximum anchor force per anchor is not divided by the spacing out of plane between anchors.
It is necessary to consider this in the anchor material parameters when using elastoplastic anchor materials. In the input box for Force, you should manually define the Maximum anchor force divided by the spacing of the anchors.
This has been fixed since PLAXIS 2D 2011.00