Hi Guillermo

I have a number of locos with Telex couplers (among which a Köf III 26340) (total approx 20 couplers)
I have battled sometimes with a very few of them, openend one and here are my findings.
They are very good couplers that work not with Relex couplers.
Never ever try to lubricate them (if you have then only 2 solutions; 1- open and clean them; 2- replace them.
On a brand new loco check if the pin can go down (w/o power) if it does spontaneously then the telex is OK (problem is the decoder / wiring)

If the pin occasionally sticks then a little bit of gym can help with a CS2 or CS3 to activate the Telex for 1 seconds des-activate for 4 seconds. Let it run some time and it will be OK

If the pin is called when you power-up, I recommend to re-initialise the loco . If it does not work, a return to Märklin could be the solution.
Here is an opened one on a brand new machine I had to repair (Märklin is not very in this kind of things)

Cheers
Jean