some times dirt can get inside them and block them, did you get no grease in

the reason I asked is most modern universals are sealed and take a very small amount if you pump till you've got grease showing you're pushing the seals out

not a good thing, also don't over do it with the slider or the grease can hydrolic lock it from compressing definitly not a good thing

since you wern't able to get any in at all then your gona need to take the grease nipples out and clean out the gallerys not to hard, soak the nipples in petrol this helps to soften them up
Good luck
Cheers Reec