Thank you for providing this additional information - as this helps greatly in understanding what is going on. I now understand (and can see) that you are using a passive microSD Card Adapter - as opposed to an actual microSD Card Reader.
Given that an SD Card is readable with the illustrated Apple Lightning SD Card Reader, your microSD cards should be readable with the same Card Reader - if the cards themselves are both compatible and correctly formatted.
As such:
1) Your passive microSD card adapter may be defective. These are very cheap to replace.
2) Perhaps your microSD cards are not correctly formatted - and are therefore not recognised by your iPad. iPad will recognise camera cards formatted as FAT, FAT32 and exFAT.
Are your SD and microSD cards the same type - i.e., SD, SDHC or SDXC and Class Rating?
https://kb.sandisk.com/app/answersweb/detailweb/a_id/2520/~/sd%2Fsdhc%2Fsdxc-specifications-and-compatibility
If they are the same type, but differ only in physically being a microSD card, suspect the passive adapter to be internally faulty, or have bad contacts.