Did you water them in well (20 liters per plant) right after planting? Were the smaller ones in a different kind of container? Were they treated any different during planting? Do they get different sun exposure?
There are many possible causes, and full details on how they were planted are needed for a hope of a diagnosis.

One sure way to kill a conifer is to plant it in wet soggy ground. Be sure the site you've chosen is free draining and doesn't retain too much water, i.e. at the bottom of hill.

Did you soak them before planting. If the compost had dried out and you planted then watered the tree has no chance. Dig them up and make sure the root ball is wet before you plant them. Did you dig out holes at least twice the size of the root ball and loosen the soil with a fork at the base of the hole and add something like bone meal to the planting mix?

Yes, the rule of thumb is to water both before and after planting. Bathgate is right, too: there's no point in planting Leyland cypress in a bog.
Another tip is to make sure that a bit of the top of the original root ball from the container is still showing once planting is complete. That keeps the base of the trunk from smothering, or growing circling roots that will later cause the tree to strangle itself.
One possibility is that the smaller plants were roughly handled somewhere between the nursery and planting, which may have broken some of the roots where they join the trunk--sort of a man-made wind rock. The bamboo stakes would have helped the larger ones resist that.

Great to have so many reply’s thank you 🙏🏻 they have all had the same planting an where watered well when planted I have 3 on 1 side an 1 on the other all the smaller all 4 are the same, yes I dug out bigger holes to plant them new soil an plant feed, I will upload a picture if this helps I was thinking maybe I was over watering or not enough I don’t know as I am new to this thanks again

Judging by the color of the foliage, the smaller ones may have been in a polytunnel until recently, to make them grow faster, so they may be a little extra shocked now.

Thanks a lot 🙏🏻 hopefully they will take to the ground soon just keep watering every night

I cannot be certain but these are fast growing trees and I think you have planted them too closely. Long term the house next door also has a tall hedge which will mean there will be competition for water. I notice that your neighbour has started his hedge a fair distance from his property. Roots of trees can be a problem near houses. I hope one of the experienced members can advise you on this because I am not an expert.

Scottsgran has a point, and I missed it in the effort to cure the immediate problem. Root competition isn't going to be a problem right away, but you may notice effects in a year or two. A couple of years more, and damage to nearby structures may occur, too.
The rule of thumb when planting a hedge or screen is to put the plants half as far apart as you want to let them get high--i.e., when planning for a three meter tall screen, put the plants 150 cm apart, and be ready to prune diligently as they approach mature height. It looks like you have planted the cypress about 80 cm apart, which means that you should trim them at around 160 cm high. Your taller ones are already a bit too tall! That one "fill in" bush shouldn't have any difficulties, though be sure to keep the shears handy so it doesn't over run its neighboring shrubs.