1980s porcelain toilet, nothing special. Recently it's developed a problem
where the front 180 degrees the bowl aren't flushed. There's plenty of
flush on the back half, and the flush works as far as clearing the contents
of the bowl, but it doesn't rinse the sides at the front.

Having a feel, there's a slot all the way around the bowl (I was expecting
holes). We're in a hard water area, but I can't find any limescale
blockages. Toilet has been cleaned with Toilet Duck equivalent which should
keep down the limescale?

A few years ago I put a Hippo water saver in it. Taking it out it's
better, but still the front 90 degrees aren't flushed.

At the hole at the back where the water emerges into the bowl, the hole
seems to have an irregularly shaped plastic stick in it, maybe 1cm wide that
goes back towards the cistern junction. I'm guessing that's what divides
the water around the two sides rather than going straight down. The stick
is loose - I can jiggle it and turn it around with my hand - but I can't
remove it because I'd have to take off the cistern to get access. It
doesn't feel like it has a naturally 'right' way to fit.

Should this bit of plastic be there, and if so is there a way to adjust it?
Or what else might be causing the uneven flush?

Thanks
Theo