Windscreen washers not working?

Probably sumat simple but my front washers arent working,(back one is fine) recently filled up, but no joy.
I remenber it started couple of months ago, they were empty so put some alredy diluted screenwash in, they didnt work for few days, then some black **** come out then they were fine, stopped working again so filled up with water this time but no joy.
Had a look under bonnet but cant see owt wrong, could it be trapped air? Any advice welcome,

Cheers

Is the pump running or making a noise, two separate pumps for front and back.

For blockages you may need to blast the tubing with an airline or something like a high capacity bicycle/foot pump with suitable cone adaptor.

There is a one way valve in the tubing, it could be blocked as well.

I have changed the standard T-pieces on the jet washer tubing to 5mm aquarium airline connectors, the jet is so strong it nearly comes through the windscreen

You can buy these for pence in any aquatic outlet, I some spares if you are stuck. The original ones become brittle and snap.

A jet washer failure is an MOT failure, the best design was in the old school VW Beetles. Jet wash was linked to the spare tyre at 42psi, simple but clever design.