Correcting the tick or as horologists say `setting the clock in-beat' is a difficult topic to address as there are a number of different types of pendulum clock; it's going to take many words to convey what would only take minutes to demonstrate, so bear with me. The key word here is `beat'. When a clock is going correctly, the sound of the ticks should be evenly spaced, i.e Tick....Tock....Tick....Tock, NOT Tick...............Tock...Tick...............Tock...Tick...............Tock.
Before we get ahead of ourselves and start meddling with the clock, I should give the usual disclaimer that I cannot be held responsible for any adjustments you attempt to make to your clock and that this advice is general and cannot be applied to every variant of pendulum clock.
Most clocks require a minimal amount of adjustment and nothing you do to your clock should ever require force, just firm pressure. If you have a mantel clock, it's possibly standing on a surface with a slight slope from left to right. For this reason you cannot turn your clock round to give easy access to the rear to adjust the pendulum; if you did so you would be setting the beat with the clock sloping in the opposite direction to the way it slopes when in everyday use.
The first thing to do is to establish that the clock is in fact out-of-beat and that it is not some other problem which is causing your clock to stop. By moving the pendulum manually to the left and right you should be able to hear a Tick in one direction and a Tock in the other. If you can hear this but the clock will not run for any length of time, try cutting some small pieces of card from a cereal box, or something similar, and slipping one or two thicknesses under one side of the clock.; 5p coins can be quite useful if you need something a bit thicker. Listen carefully to the tick to see if it sounds more even. If it sounds a little better but not perfect, add some more slips of cardboard, if it sounds worse, or there is no tick at all, take out the cardboard and put it under the other side of the clock and be prepared to experiment with the amount of packing to gain the best results. Always lift the clock very gently when the pendulum is attached and avoid sudden movements. If this sorts out the problem and the clock keeps going, you have the choice of either leaving the packing in place or getting your courage up and tweaking the pendulum!
If you cannot get easy access to the rear of the clock this is quite challenging. Let's imagine your clock stands on a broad surface and you can comfortably get to the rear. Remove your packing and open the back door. Gently stop the pendulum and slowly move it to the left and then the right a few times and you should be able to sense that you move the pendulum more to one side to get the clock to tick than you do to the other side. Note which side requires the furthest movement from the centre position to get the tick. Let's say you need to move the pendulum more to the right to get the clock to tick. In this case you need to move the pendulum gently to the LEFT until resistance is felt and then move the pendulum slightly further to the left applying a little pressure to do so and then see if this has changed how far the pendulum needs to be moved from the centre line to get the clock to tick. Some clocks need very little extra pressure to reset the beat, others need more and you may find that you move the penduum too far to the left then have to move it to the right and apply a little pressure in this direction to make the correction. Even experienced clock makers can spend considerable time trying to get the beat correct.