Hi @Mandy-G,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
It sounds like you have everything you need, so let's run through some troubleshooting steps.
You would start by removing the black cap from the opposite side of the entry connection, then attach a threaded tap adaptor to the opposite side of the sprinkler, which you can then connect your additional hose to, which would then connect into the next sprinkler in the same fashion.
Assuming you are doing this correctly, then I would be checking inside the sprinkler to see if there is a seal or anything that has to be removed to allow the throughflow of water. If you can't see anything, maybe try putting one of the other sprinklers first in the chain to see if the issue is specific to that sprinkler, in which case it can be returned for a replacement.
If this doesn't appear to be the issue, check the hose connections to see if anything is blocking them. There could be a misaligned washer or some debris blocking things.
If nothing is blocking the sprinkler or hose connections, then it could be a pressure issue, in which case you may need to speak with a plumber to see what can be done about increasing the pressure. A Flow Rate Test can give you a good indication of what your pressure is like.
Give these solutions a try and let me know what you find. I am happy to assist further if required.
Jacob
