I had this problem just the other day. Somehow your constraint options are unselected (think autocad Osnap settings). in sketch mode, RMB on the window and select Contraint Options, then select ALL.
Access: In a sketch when no command is active, select Constraint Options from the right-click context menu.
Selects the constraints to be inferred. Selects (scopes) the geometry to which the constraints are inferred. The scoped geometry is shaded differently after the dialog box is closed, unless all geometry is selected.
Selection for Constraint Inference
Select All: Places a check mark in each active check box.
Clear All: Removes all check marks. No constraints are inferred.