Land drain not working

[Mark Dog Boy]

My back garden is waterlogged every time it rains. There is a ground drain running parallel to the house and right under the worst part of the garden.
Theres an inspection hatch and I can feel the ground piper entering it ok but there's no water flowing in even in heaviest rain.
ive lifted the topsoil over a part of the drain down to the lining around the drain. The hole can fill with water and appears not to drain down into the drain.
Any ideas what could be wrong or what I should investigate next would be great
thanks

 

[rocketmanbkk]

It's blocked & needs attention.