Always ensure you are using the latest LendingPool address, since Kovan may be updated from time to time. You retrieve this by getting the latest LendingPoolAddressesProvider, then getting the latest LendingPool address.
For assets on testnets, we use different versions of the token (e.g. testnet Dai). This is to ensure enough liquidity for our reserves and to easily mint more tokens when needed.
If you are developing on a testnet and require tokens, go to https://testnet.aave.com/faucet, making sure that your wallet is set to the relevant testnet.
As testnet assets may change frequently, ensure you are using the right asset by minting from our testnet faucet (as mentioned above) and use the ProtocolDataProvider to fetch all the relevant assets using getReserveTokensAddresses.