It probably is length. Some jack leads have fatter shoulders than others or a chunkier base plate and don't go all the way in.

I imagine that your guitar has an endpin jack socket. Check that the end of the threaded bit of the socket itself does reach all the way through the cover plate and make sure the cover plate is well screwed down. You may need to adjust the socket a bit. They usually have a threaded length that fits through the tailblock.

Remove the coverplate (the bit the strap goes on). You will see the securing nut and a washer - and the threaded shaft usually with a cross-hole so you can stop it going round. Put an allen key through the cross hole to loosen the nut. Do not remove the nut yet.

Remove the allen key and then remove the loose nut and washer with your fingers TAKING GREAT CARE NOT TO DROP THE THREADED SHAFT INTO THE GUITAR.

Now take a jack lead with a slim jack plug and unscrew the cover of the plug and slide it back out of the way. Carefully holding the threaded shaft plug the jack plug in.

Now using the jack plug poke the threaded shaft in through the tail block and with a bit of wiggling you will be able to juggle it out of the soundhole - wire attached going to the undersaddle pickup and wire attached to the jackplug. Back the nut on the shaft off a bit so the threaded shaft will go a bit further through the tailblock.

Now hold the guitar upright over your head and by dangling the wire from the pickup and drawing on the jack lead you should be able to get the threaded shaft back through the tailblock. ALL WITHOUT TAKING THE STRINGS OFF!

Replace the washer nut and cover!

In the meantime take your own jack lead with you to open mics!