I had this issue like 3 days ago..
get a bottle cleaning brush...
air compressor if you have one if not one of those cans of air that you use to clean dust, some wd40...
blow all the dirt out near the plug which there is always loads of due to a bad design...
soak the brush in wd40 stick it in the hole and push it up and down and turn it around...
spray some wd into the hole to make sure its all come out brush it some more..
cover new plug in oil and try again...
brush can be bought from poundland, they have a big end and a small end on them, small end fits fine.