Glued stair steps are coming loose

Hello everyone, we moved in mid-last year and have a steel staircase with wooden steps. The wooden steps were only glued at that time. Now we have the problem that since we moved in, 2 steps have come loose. The carpenter used Lugato bombenfest as the adhesive. A kind of assembly adhesive. Can you recommend something better? The steps have been oiled twice and were not scored or anything similar in the glued area. (We had seen that with the first step.) After the first step came loose, we reglued it at that time. The second one is not completely off yet, but already loose. The glue holds on the steel but not on the wood. What would you recommend now? I have attached pictures of the staircase construction for you. Of course, there is the possibility to screw the steps as well. However, we did not want to see screws from below. If you screw them now, is it possible to drill from below and keep the steps in place?

Old painter's saying: nothing sticks to oil. That is the first problem. And wood moves. Metal does not. This is a very, very unfavorable combination regarding the durability of the connection. Who recommended this to you?

Yes, I suspect that now too. I wasn't there when the steps were glued. I had actually thought that they would be sanded down.

We did say that we wanted to glue them. I know this from my parents. The frame is identical, but the steps are not oiled. It has lasted for 15 years. The carpenter did not object and got everything needed for the fastening.
With the first loose step, I noticed that about half a millimeter of "wood material" remained on the glue. This mass of oil/wood then came off the step. So that is the weak point, as you also write.

Are there adhesives that might penetrate deeper into the wood? Or is sanding the glue surface on the wood sufficient?

If wood came off with it, then the glue is so strong that it tears the wood fibers with it. Then the poor adhesion due to the oil is not the sole problem. When wood moves (which it always does, especially with oiled, i.e. open surfaces), it detaches more easily at the oiled spots and tears off at the spots with less oil. This cannot be avoided. Screws would then cause the steps to crack. The staircase at the parents' was presumably varnished?

I'm not sure about my parents. If I remember correctly, we have beech steps that were oiled twice for the current color.

I don't know exactly about my parents. They are not painted. I would rather say they are oiled. The staircase was heavily used with 3 kids, with jumping down, sliding, and what else comes with it. Everything is still firmly in place like new.

For us, a friend who admittedly goes down the stairs strangely (jumps) and a big sturdy tiler are enough to loosen the steps.

If wood came off with it, then the glue is so strong that it tears off the wood fibers. Then the poor adhesion due to the oil is not the only problem.

I will loosen the step later and send a picture of it. Real wood fibers did not remain stuck back then.

Does it look more like oak?