Mower deck Ferris IS5000 Mower leaving rows of grass.

I have a Ferris IS5000 mower, which has developed a problem with the grass clippings. The mower cuts perfectly, but has developed a fault, so instead of the grass clippings being spread evenly out the back, it is leaving, a huge heap of grass clippings, that are coming out the centre rear of the deck. I have made no changes to the mower and have absolutely no idea what the problem can be, especially as the machine is cutting so well. The deck belt has also recently been changed and the blades are new, any ideas or suggestions, as to what the fault may be, would be appreciated, thank you.

Did you have the problem before the deck was serviced???
olf20 / Bob

The mower was working fine and nothing has been touched, the mower deck just seems to be throwing the grass out the back in a huge swath, rather than spreading evenly out the back of the machine. We had three and a half months of very dry weather in the UK in the spring, but thankfully the rain has now came and everything is growing again. The issue could have been happening early on in the spring, but with the lack of grass, due to the dry weather, the problem wasn't noticeable.

The only thing I can think of is there might be grass stuck to the bottom of the deck and are you sure the right size belt was installed...