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There were still 25 unhatched eggs in the incubator 4 days past the hatching date.....
But when I had a looked inside I found fully formed chicks still with a part of the yolk attatched......Should I have left them in there longer ??? Is this normal or unusual ???
Or could the incubator be faulty ???

Usually, if you find fully formed, unhatched chicks in the incubator that far past the hatch date (assuming you have calculated it correctly based on species), it means that something occurred late in the incubation process that killed the chicks before they could hatch: a serious temperature spike, a lack of oxygen from poor ventilation, extremely high humidity (causing the chicks to drown in the shell), etc. Very careful and close monitoring of your incubator conditions during the last few days before hatching can cut down on such problems, but a disaster can occasionally happen to anyone.

Sorry to hear about your losses. Better luck next time.

Another thing that can delay hatching is your incubating temperature being a little on the low side. If your chicks were definitely dead when you broke open their shells, it sounds most likely to be humidity too high for the whole of the incubation process. Eggs being porous do need to lose water from within the shell or it will drown the chicks when they peck through into the air sac.