I always drill a small pilot hole in masonry first. This not only guides the larger bit in, but also leaves a smaller hole to repair should it turn out to be an unsuitable spot for the hole.
I also suggest drilling through a mortar layer without the hammer on, so that it goes in straight and cleanly. Then switch to hammer for the masonry layer. If the hole is a large diameter, you may need to open up the guide hole a little to not overload the large drill bit's cutting capacity. - TedBear
You shouldn't ever need to put a lot of force on a masonry drill for it to drill correctly. The idea is that gentle pressure on the drill unit itself will allow the tip to bore into the material and the flutes in the drill bit to clear the material in the hole as it is drilled.
Putting too much pressure on a masonry bit will ruin the temper (by overheating it) of the carbide tip and make this blunt, rendering the whole drill bit useless, it is that tip that does all of the actual drilling. SDS bits are not cheap and ruining one by mis-use is simply poor economy.
Also, most SDS drill motors/units have a variable speed trigger, if you start off slow, so that the drill bit doesn't jump all over the place, sometimes "pulsing" the trigger gently, the tip will eventually sort itself out and start to bore into the material, just take it quietly until you know that the drill is about 5-10 mm into the hole.
I use a masonry drill quite frequently as an Electrician and I offer the following advice: Let the drill tip do the work.
Be aware that there may be steel reinforcing in the material you are drilling into, this can potentially destroy your drill bit tip, if the bit starts to bend, stop drilling and reverse it out and check the tip, the sides of the tip should look like the picture of the new tip that Mitchell posted above.
You could possibly drill out the reinforcing with a HSS drill bit and some cutting oil, this is very risky and can cause the drill bit to snap in the hole when it contacts the rebar.
Don't ever try to cool a masonry drill bit with water as it is drilling, this is not how they work, they need speed and temperature to cut through the masonry, you can (if you're drilling a large enough hole) end up with steam burns and a wrecked tip at the same time.
What I have done in the past, when I have struck steel reinforcing members in a wall, is get a HSS twist drill bit of the same size hole and coat the end of it with cutting oil (Something like Rocol RTD) and drill out the rebar (remembering to take the "hammer" function of the drill off before doing this).
It will be hard work, do not force the drill bit into the hole, but you will get through it if you keep the tip of the drill wet with cutting oil. Also bear in mind that when the rebar is broken, the bit may catch on the ends of the pieces of steel, so be prepared for that.
A lot of people never even expect to strike reinforcing in a wall inside a house when drilling into a concrete wall.
Chances are, no matter where you drill, 40-50% of the time (if the wall has decent reinforcing in it) you will strike it.
The first sign of this is when your drill bit no longer penetrates the wall, with light pressure on the drill unit itself (which you should always use anyway), it will just stop and go no deeper.
That is why before drilling into any concrete/masonry wall you wrap the shank of the drill bit with 1 or two layers of PVC insulation tape, near the chuck of the drill, this gives you a visual reference of how the drill-bit is travelling into the material.
Should this stop, back the drill bit out, drilling any further is going to destroy the tip on the masonry drill, when it heats up against the steel and will often start to squeal, that is too far and the tip is then damaged.
Tungsten carbide tips on masonry bits are only made for cement and brick, not steel. They are strong, yet brittle and will destroy themselves when over-heated. - MikeTNZ
