Hi and welcome to the forum,
I'm taking it for granted that your ceiling is made of plasterboard.Most cracks that run straight are caused by the joints in the boards.Push up on the ceiling along the length of the crack and see if there is any movement in the boards.If there is, you will need to put 1.5"screws into the joists,either side of the crack,along the whole length of it,and see if that helps.This is the root of your problem.To put a piece of flimsy fibre tape over it and cover it with probably a thin coat of plaster without doing that first, was a waste of time and money,and would never really work.....Apart from tightening up the boards,I don't think you could fix the ceiling yourself.I think you would need a plasterer to come in and do it all over again,(if you wanted that) but this time use 4" wide jute scrim to cover the crack,then cover the whole ceiling with British Gypsum bonding-coat.This is undercoat plaster, apply 2 coats.The scrim would be bedded into this undercoat plaster along the whole length of the crack.When all of this has set,the ceiling would get 2 coats of multi finish.This might seem a fair old thickness of plaster overall,but it is only 5-6 mm, if that.You will get a far stronger job.I do it this way all the time.You would need to PVA the ceiling of course before any plastering takes place.The plasterer would do this on the day.The whole plastering side of your job should be done in a day, and the key to the success of your job,are the screws,and a decent plasterer.
Roughcaster.
