Why won't my first layer stick?
Usually nozzle height, a dirty bed or wrong temperatures. Fix them in that order, then slow down and add a brim.
Usually because the nozzle is too far from the bed, the bed is dirty, or the temperature is off. Fix them in that order.
1. Nozzle height
The first layer should be gently squished onto the bed, not laid on top like a round string. If lines look round and separate, lower the nozzle slightly with Z-offset. If the nozzle scrapes or the plastic looks transparent and thin, raise it.
2. A clean bed
Fingerprints leave oils that stop plastic sticking. Clean the build surface as its maker recommends, often with isopropyl alcohol or warm soapy water, and avoid touching it afterwards.
3. Temperatures
Check you’re using sensible nozzle and bed temperatures for your filament. The spool label usually gives a range. Start in the middle.
4. Slow it down
Print the first layer slower than the rest. Most slicers have a separate first-layer speed.
5. Help it hold
A brim adds a thin border that increases grip and reduces corner lifting. Turn off cooling fans for the first layer or two.
This is Lesson 12. See the whole of Unit 4 on the lessons page.