Damp issues in underground basement

[Tillyfest]

Hello in need of some help

I have recently had damp issues in an underground basement. The house is at the bottom of sloping ground.
The original external rear of house wall was plasterboarded which then became mouldy, I have removed all the plaster board tanked and rendered the walls and there is no damp however the concrete steps leading down to basement which I did tank and screed appear damp I have removed the screed on a step to check and it's got black mild and the original concrete step is damp to the touch

There are no pools of water and don't have condensation I feel it maybe run off water issues or a water leak underground. The new screed has a small amount of Brown discolouring.

Any idea how to solve the damp concrete step issues

Thanks steve

 

[bobasd]

ive asked mods to transfer this post to bldg.

you would get better advice if you post a few photosof where youve tanked on the ground floor and the basement and photos of the wall outside.
can you show photo of screed on steps? is the screed on the treads?
whatt do you mean when you say youve tanked an rendered?
was the plasterboard dot and dabbed on?

 

[Tillyfest]

A stud frame was fixed to original external wall with a black plastic sheet then plasterboarded fixed. The plaster board showed signs of mild and when removed the sheet and wall behind were soaked. I filled all the cracks in the mortar then tanked the walls with 2 coats of tanking slurry. When dry, lime render was plastered over the tanking.
There has been no further damp issues with the walls.
As the steps were cold and damp I tanked the steps at the same time as walls thinking a concrete screed could just go over the top of the steps
After 3 months the steps appear damp and the screed and tanking on a few steps have failed. I removed the screed and found black mould on the steps and the rear of the damaged concrete I removed from the step

Water must be rising through the old steps but how do I rectify it ?

're tank the steps and screed or other ?

Thanks steve