Based on this answer:
If I remember correctly; The Slim version supports only the PS/X(1995-1999)version memory cards, not the PSOne(2000-2500) version ones. This could be your problem.
- Are there actually different versions of the PS1 Memory Cards?
- What are the difference between them?
- How do you identify the different versions/revisions?
- Are there any forward/backward compatibility problems? (I see that the PS2 Slim BC is one)