Problem with changes - Mitsubishi eclipse 98

Good morning, I have a 1998 Mitsubishi Eclipse. The problem is that at certain times, the downshifts (second, fourth, and reverse) stop engaging. After several attempts, they might eventually go into gear, and when one of them engages, they all work smoothly again until the problem reappears. It's important to note that the transmission doesn't make any noise when I try to shift gears, and even if I turn off the engine, it still won't go into gear.

Hmm, I'm checking the gearbox support, in case it's tilting. Also, the selection is by cable, right? If so, check the cables, especially the ends that go into the gearbox; they have a rubber center that sometimes comes loose. Check it and let me know.

Thank you so much for your reply.
I've already checked everything. Today I disconnected the connectors that run from the shifter to the gearbox and tried to move the mechanism manually, but the same thing happens; the ones that go upwards only engage. Clearly, the problem is inside the gearbox, but I don't know what it could be or how complicated and expensive it would be to repair. Perhaps it's just a loose or misaligned part, but unfortunately, there's no other option but to remove the gearbox.

Damn, you'll have to take it out. Maybe the inner leg that runs through the channel is damaged, but you should find it at junkyards. Take it apart first before saying anything. There are some little balls that can also come out, fall off, or get ground up. Check it and let me know.

We'll see what I find; I'm going to take it apart next Monday.
It would be a great help if anyone has the parts diagram for the gearbox that comes with this car.
Regards, and I'll share any updates on the forum as soon as I have them.