It's physically impossible to fall or slip "between the cracks"!
The correct way to say it is to fall/slip through the cracks. This expression is used figuratively to convey that something or someone is being overlooked or missed.
For example:
Some COVID-19 patients can fall through the cracks as healthcare systems deal with the overwhelming nature of the pandemic.
In Spanish, "fall/slip through the cracks" may be translated as "pasar desapercibido/a" or "pasar inadvertido/a."
In Catalan, that can be "passar desapercebut/da" o "passar inadvertit/da."
Using the example above, we can have the following translations respectively:
Spanish — Algunos pacientes afectados por la COVID-19 pueden pasar desapercibidos a causa del carácter apabullante de la pandemia al que se enfrenta el sistema sanitario.
Catalan — Alguns pacients afectats per la COVID-19 poden passar desapercebuts a causa del caràcter aclaparador de la pandèmia que afronta el sistema sanitari.
Besides writing and editing medical documents in English, I can also translate them into Spanish and Catalan.
If you're looking to communicate your content properly and clearly, I'm happy to help!