My layout must be getting towards a hundred yards of N gauge track and I've not found the secret yet! All I can suggest is:
When breaking the strings of four into single joiners, if like me you bend them up and down then keep them below the "horizontal" if you see what I mean. Otherwise when they break off the broken bit may be bent upwards and blocking the place the rail needs to be. If it is bent upwards it can probably be bent down with small pliers.
Make sure the rail is not burred, especially after cutting it and especially underneath.
If they won't go on by hand pressure try either pushing them on with a small piece of wood or fitting them with small pliers. But use the pliers to grip the very end of the joiner where it is only a single thickness of metal, otherwise you'll squash them together and the other rail won't go in.
Put the joiners on each piece of track before you fix it down. Slide the next piece of track into it horizontally not down from an angle, otherwise the joiner will be bent upwards which may cause a dip in the track. It may help if you make one rail end stick out a mm or so more than the other one, so you can get one joiner correctly aligned then worry about the other one. You can use the small pliers to align the joiner as the rail goes into it.
And finally, if you haven't already, then invest in some small pliers!