There could be many possible reasons for that.
Many exchanges have various restrictions on API connections. You can see some of them here. For example. Delta will not import transactions for coins with 0 balances and 0 withdrawals and 0 deposits on Binance.
It may occur because deposits and withdrawals are adding to or deducting from the balance even though they were just transfers. For example, if you only add Binance Connection to a portfolio and at some point you transferred 1 BTC from an exchange (for which you don't have an exchange account connection) or a wallet to Binance, the Binance Connection will import a 1 BTC deposit and add 1 BTC to your holdings. You may have already accounted for this 1 BTC as a buy transaction on the exchange. In this case you will need to manually edit the 1 BTC deposit entry and change the source of the BTC from
Other/UnknowntoExchangeorWalletso that it is treated as a 'Transfer' and not a deposit.