Try lowering the maximum z-axis speed and accelerating over a longer distance. You can adjust these settings from within the web application on the device page > hardware widget.
Check the eccentric spacers on the cross-slide that are used to adjust the v-wheels that support the z-axis extrusion. If the eccentric spacers are adjusted too tightly, then they will exert too much force onto the z-axis extrusion which can make it difficult or impossible for the z-axis motor to raise the z-axis.
It is possible that dust and debris can build up on the leadscrew causing additional friction between the leadscrew and the leadscrew block. Clean the leadscrew with a rag and then add some lubricant which can be purchased from a local hardware store.
Do not lubricate belts and pulleys
In case you are reading this document and also have issues with movements on the X or Y axes, do not take this piece of advice (“Lubricate the leadscrew”) and apply it to the belt-driven axes. Applying a lubricant to the belts may cause a chemical reaction that rapidly degrades the belt material. Furthermore, a lubricant between the belts and pulleys may cause the teeth engagement to slip.
Try adjusting the z-axis stepper driver to deliver more power to the z-axis motor using the instructions at the bottom of this post.