There are two options for treatment of dividends in Portfolio. Dividends can be paid directly into your cash account for the Portfolio. They will show up there as individual transactions. This is the default. Or you can have the dividend reinvested in the purchase of additional shares. Each dividend will be used to increase your number of shares, including partial shares, for that holding.
The dividend management treatment you choose is valid for the date when you select it, so if you change to reinvest dividends, they will be reinvested from the date you make that change.
To view these transactions, select "View Transactions" from the drop-down menu on the right-hand side of the row for that holding.