The mac version of the app didn't recognise the seed of android version - and same with the linux version of electrum.
Edit: The problem was fixed though when updated the mac version from electrum-3.3.3 to electrum-3.3.4.
ps. There is a way to export a private key on electrum android app:
first make an invoice in send tab, then click the invoice, then click 'Details' and now you can choose 'Export private key'
Except it gives an error!