Have you tried to fertilize the plants in spring ?

Your garden geraniums have never bloomed in two years.
That suggests something is being done by you or the plant is unable to produce flower buds.

I suggest two methods of trying to rectify this.
First, have you been feeding your plant a high nitrogen type fertilizer---i.e. the first number on the package of fertilizer...is it much higher than the other two numbers.
Nitrogen...the first number...encourages strong growth and greeness of color....but does all this at the expense of bloom. Hence, Miracle Gro...has its ratio of elements 15/30/15....much higher in phosphurus which encourages strong roots and bloom.

You might also be giving it high nitrogen without you even knowing it. Is the garden near the grass of your lawn and do you feed your lawn ---especially in early spring.
Early spring fertilizers for lawns has a high nitrogen content...to encourage the lawn to green up and grow strong.
The nitrogen can leech out into nearby soils.

Then again, do you mulch with ...evergreen needles ..such as pine or spruce needles. These needles are high in nitrogen which can act as stated above. Better to not use pine needles where it isn't needed.

Another possible reason in the same vein....do you heavily mulch your beds with wood chips...and do you replace every spring that much mulch that disappears.
Wood mulch in the garden will break down..and uses nitrogen to effect it. A garden devoid of all nourishment might be the cause itself to not produce flowering plants that otherwise should bloom.

A second way to encourage a plant to bloom is to put it under stress.....cause it to want to protect itself by producing seeds....to promulgate the species. Seeds come from blooms...so if you can cause your plant to think its under attack, it will bloom in order to produce seeds.

With a sharp spade, go outward of the shrubs branches and make quick sharp stabs into the ground to hopefully cut the feeder roots. This causes the plant to think its under attack....and will (can) cause it produce bloom.
This is more likely a way to encourage a tree to bloom rather than a small shrub.

what does deadheading mean and how exactly do you do it?

Deadheading means pulling or breaking off spent blooms. Most plants try to reproduce. They do this by making a blossom, which, as its petals fall off, makes a seed pod. If left on, the whole plant has a tendency to die back, or go dormant. If you break off the dead blossoms, the plant will continue blooming.