What hard thing am I hitting in my brick wall?

Question

I'm drilling several 2.5cm deep holes (all horizontally about half a meter from the roof) to install some large IKEA Pax cabinets into an internal single-brick wall. I'm drilling into a wall with a small masonry drill bit to make pilot holes before working up to bigger bits.

I drill about 1.5cm in to each brick with ease, and then I feel like it doesn't go through any more. I try push hard and try using the hammer function, but it doesn't move.

I've checked the house plans and photos of the wall, there shouldn't be any pipes or cables in the wall. All pipes and cables appeared to be run vertically during construction. I wouldn't think there's a horizontally running cable or pipe. Most of the information online is in regards to drilling into drywall with suggestions of hitting metal plates, however, I didn't notice anything like that during construction, and I presume that's regarding drywall construction. The cables I noticed during construction run vertically up from power points via lines drilled into the brick itself (not conduit).

Only 1 of about 10 of the holes were successful (unless I got "lucky" and drilled into mortar instead of brick?)

I'm nervous to dry to drill further. In-case I hit something I shouldn't, even though all the above tells me that there shouldn't be anything there.

Am I just hitting the other side of the brick inside the air pockets of the bricks? Certainly it shouldn't be harder to drill into the center of the brick than the face of the brick...or is it?

This is a new house in Australia. I believe the bricks are a single layer like the below:

Photo of the wall during construction wall looks like this:

From the floor plan:

From the electrical plan:

Answer 1

Are the walls plastered and finished?

If the answer is yes, then it is possible the 1.5cm you've been able to drill is in fact just plaster and board. This then further leads to the possibility that as you suggest the successful hole is in a mortar line. You've not said if you have previously had experience if drilling brick or not, but basically brick is much harder than we think.

Related anecdote:

I was recently drilling in to my brick-built chimney breast. This ended up being much more difficult than I originally anticipated, and I've drilled in to brick walls before, although as this was for large bolts I was using a much bigger bit than I am otherwise used to.

What can you do next?

Helpfully you have photos of the types of blocks used to construct your wall. What I would do next is acquire one or two from somewhere and then drill holes in them to judge the difficulty and the force needed. If they are easy to drill, you know you have another problem. If they are difficult you know it should hopefully just be a matter of patience.

A point about drill "hammer" functions:

Not all drills which have a hammer function operate in the same way. In my anecdote above, I started with a relatively light-weight battery powered drill. The hammer function on that makes a lot more noise but otherwise doesn't do much useful. I changed to a corded drill which uses a lot more power. The hammer function on that makes a frankly terrifying amount of noise, but it also created the holes I needed. So the type of drill you are using may also be a factor.