You picked the wrong plant! You do know that all parts of oleander are toxic!. Never let kids or pets get a bite of it! Be careful where you toss the clippings and don't burn them!!! I'm sure your local nursery can suggest a great replacement plant that will give you the cover you desire.

Looks like you're pruning it to a very flat shape. You won't get coverage with an oleander unless you let it get wide and bushy. The leaves are too thin and the plant isn't brushy. This isn't the right plant. Also agree with the @decoenthusiaste.

Also too many plants crammed into too small of a container(s). Gardening in containers is a natural dwarfing/stunting process anyway but root congestion and competition will exacerbate the condition and prevent the shrubs from becoming full and densely foliated. Routine fertilizations is necessary and will help but won't fix. You need different plants and bigger (wider and deeper) containers.

I've never seen oleanders grown in pots or pruned so precisely so I don't know how they would respond to more pruning--I hope someone can help you. It's a really interesting use of oleanders. I bet they're very nice when they bloom. Still, a happy oleander is one that can be the size of a Volkswagen and potted lifestyle might not be a good fit for this plant.

(Oleanders are very popular plants where I live and I can't find and never have heard even anecdotal reports of people or pets being sickened or dying from them. The danger appears to be overblown.)

https://csuvth.colostate.edu/poisonous_plants/Plants/Details/60

Facts on oleander toxicity

You might enjoy a good read titled "White Oleander."

IMO the wrong choice for your needs .Picture showing the whole area could be very helpful.