We are using the API provided by etherscan.io and ethplorer.io in order to import Ethereum Wallet related transactions. These are the best and most reliable APIs on the market. However we are only able to import what they provide over the interface. So unfortunately not all tokens are supported yet and there are some requirements to fulfill, before they list tokens in their API.
In some cases it is necessary to enter the data manually. It's easy to find this data on Blockexplorers like etherscan.io.
For example there might be failed transactions are not imported via API, but the fees for them have been deducted from the balance. They have to be added manually as transaction type "Lost".
You can enter missing transactions either manually on the Enter Coins page or via the Excel Import or the Custom Exchange Import.
In addition we provide CSV imports for some software/online wallets (e.g. Trezor, Exodus, Electrum etc.).