Why does my fertiliser injector not work

[ArronOB]

Hi. I have a small shadehouse with an overhead watering system. Just a 12mm mainline directly from a garden tap, a couple of short branch lines, and a few spinner style spray heads. I wanted to be able to inject some fertiliser into the line so I bought one of these

1/2 Garden Irrigation Device Venturi Fertilizer Injector Switch Water Tube KitZ5 193571793492 | eBay

It doesn’t work. No Venturi effect is created and no fertiliser is drawn into the main line. I tried different water flow rates and other variations and alternated directions but without result. Can anyone suggest what I might be doing wrong ? ?.

Cheers
Arron