"Gridlines" ? I assume you mean guidelines (could have been grid).
Remember that to select an object, the layer that the object is on, must be your current layer. Sometimes you may create a guideline not realising that you are on a different layer to which you intended. For this reason, I prefer to create a layer and call it Guidelines, and move all my guidelines to that layer.
The problem with selection can also be that the software is aware of several possible objects that you could be selecting, ie if selecting a guideline that is placed over a room, the software will also attempt to select the other objects such as floor, roof, ceiling, grid and even another guideline, that may also be under the cursor. When this occurs each click at the same point will cycle through each object in turn. To avoid this, try selecting a portion of the guideline that can not be confused with another object.
Alternatively switch off objects that you currently have no interest in selecting. To achieve this, Right click on the Grid tool, and the the General settings dialog select the Selection side tab,where you can select / deselect the different objects.
In the same dialog, in the Snap tab, you can also change the snap radius and the snap attributes of other objects.
If you want to snap to a guideline, try switching off the grid in case you are snapping to that.
Zooming in also helps when selecting guidelines.
