I have covered PPA in classes before and the children often seem to have a different relationship to a covering teacher even if they see them regularly than they do with their regular class teacher.
It can also be tricky teaching in someone else’s classroom as you may feel like you’re entering the children’s space as they know where all the resources are, the specific routines for that classroom etc rather than inviting them into your own teaching space.
The best advice I can think of is to find out about the class before you teach them from their class teacher (e.g. do they like competitions, work best in pairs etc, What about individuals with specific needs - what do they need - respond best to? Who works well together and who doesn’t?)
Set up your own routine when you come into the room and stick to it (e.g. starter up on the board or even a question for them to discuss to settle the children whilst you speak to individuals.) Repeat for end of lesson. This might give you a minute with some of the quiet children or tricky individuals to help you build relationships.
Use the school’s behaviour system and always follow up including positive rewards too. Try to follow the same system as the class teacher e.g. warning, then is it name on the board or do they use initials or does the teacher do something else? Some teachers have table points or whole class rewards so good to find out about this too and use it to your advantage.
I carry resources I use in a big plastic pull along crate. You could put in spare pencils, whiteboard pen, paper etc. I find if I’m really organised that I feel a bit better about going into tricky classes. They also won’t have the excuse that they don’t have a pencil so can’t do the work.
Hope some of these suggestions might help. At the end of the day it takes time to build up a reputation / rapport with classes especially if you cover them once a week. I think every PPA teacher I’ve spoken to has at least 1 class they struggle with. One teacher friend advised me that with these classes pretend that they are your favourite to help build up a positive relationship.