You need 10 KMD of interest earned in order to collect it. Had you earned 10 KMD when you recently tried?
Not now. Ok, will wait

But in the past I had once only about 11 KMD balance in my wallet and could collect interest amounts of 0.0xxx.
Thought it only had to be 10KMD or more in the wallet to get interest. For collecting interest, there is a limit, too?
I will repeat the interest rules for the 47th time:
It is based on utxo (search on google if you dont know what this is)
If a utxo is for more than 10 KMD and older than an hour and has nLockTime set, it accrues interest up to a max of 5% at a rate of 5% per year.
To collect it, you need a wallet that understands the accrued interest so it can spend more than the inputs (that is a violation of normal bitcoin rules)
The above are the rules that are enforced by the blockchain (and not any specific wallet)