Upper shelf still dirty after washing

Model Number: df61
Brand: Hotpoint
Age: More than 10 years

Although my dishwasher is old it works perfectly on the bottom shelf. I have over the years looked after the machine, buying replacement bits and pieces as its been needed. I suspect my problem is to do with my last replacement purchase. I needed a new Upper spray arm. I was told by the engineer that Hotpoint had stopped producing the one I wanted and that the updated arm would work just as well. I suspect the water is not travelling through the pipe at the back as there doesn’t seem to be a water tight seal between the two parts. Can anyone tell me whether the original arm in still in production or whether there is a cheaper solution to my problem.

Many thanks,

Jackie 🙁