This looks like a bug in the plugin. I would suggest to open an [issue report](https://github.com/borysiasty/pointsamplingtool). Please note that gis.se is not a bugtracker and it is unlikely that this post here will help to fix this issue.

For further possibilities to proceed, please have a look at [this answer](http://gis.stackexchange.com/questions/144852/qgis-numerical-vertex-edit-anyone-else-finding-it-doesnt-work/144864#144864).