Print not sticking to build plate

When you're experiencing difficulties with prints sticking to the build plate, there could be a few things going on. It could either mean that the first layer just doesn't stick well enough or that the prints gets loose due to "warping" of the plastic. Warping basically happens because of the properties of the plastic. Plastics have the tendency to shrink when cooling down fast (some plastics more than others), which could eventually lead to your print curling up (at the corners).

There are a few things you could do for a good first layer of a print and to reduce the amount of warping.

Bed leveling

For printing it is very important that the first layer is nicely squished into the blue tape and sticks well to it. If the distance between the nozzle and build plate is too big, your print won’t stick properly to the glass plate. On the other hand, if the nozzle is too close to the build plate it can prevent the filament from extruding from the nozzle.

In both cases this will lead to a bad first layer, which might eventually lead to adhesion problems. Make sure to re-level the build plate if you're experiencing this issue.

Blue tape

After a lot of printing, the blue tape that is on the glass plate may wear or damage a little bit. This can cause an uneven print surface or bad adhesion to the print and it is therefore recommended to replace it once in a while.

Using a "Brim"

A nice feature in Cura to help preventing warping is called "Brim". It will place a single layer thick flat area around your object, thus creating a bigger adhesion surface. The Brim can easily be removed once the print has finished.

This option can be selected when using the Full settings in Cura. You can find it at "platform adhesion type" in the basic settings tab.