How would I stick this back up?

[purpleroad]

 

[EFLImpudence]

It screws on.

 

[purpleroad]

It screws on.

Keeps falling off though. Will try again

 

[John D v2.0]

Maybe either the threads are broken on one or other end, or the ceiling was skimmed after the fitting was up.
Or perhaps it was broken and replaced with a non matching one.
If it really won't stay, and you're confident with electrics, isolate the power completely and take a notE of the wiring, then replace the whole ceiling fitting with a matching one.
Only a few pounds.
Edit; fix confusing typo

 

[purpleroad]

Maybe either the threads are broken on one or other end, or the ceiling was skimmed after the fitting was up.
Or perhaps it was broken and replaced with a non matching one.
If it really won't stay, and you're confident with electrics, isolate the power completely and take a not of the wiring, then replace the whole ceiling fitting with a matching one.
Only a few pounds.

A not of wiring? I don't understand, sorry

 

[OwainDIYer]

A note of the wiring, i.e. record which wires go where.

 

[John D v2.0]

Yeah photo would do, or a drawing. I'll edit my typo.
Should have perm live (3 core), switched live (1 core), earth (3 core) and neutral (2 core) plus the lamp will be fed off the switched and the neutral.
Its possible to have one less of all except the switched live.

 

[securespark]

The most common issue here is the thread on the plate getting gummed up with plaster or paint. Ensure the thread is clear of obstruction then try again.