Why are my billable items not populating when I create my invoice?

The first thing to check is if your entries have been validated. On the T&M Overview page, the daily graph will show either a red or green dot. If the dot is red, then the day has not yet been validated and therefore those billing entries can't yet be added to the invoice. If this is the case, you can validate the days by going into the T&M Billing section, clicking on Actions, and then choose Validate Days. Here you can select the dates you want to validate and submit. 

Once the days have been validated, navigate to the invoice section. When you create the invoice, ensure that the date range covers the days you want to invoice for. Then click into the empty Product cell and at the top, select T&M. Then you can choose the category and all billable items from that category and date range will be captured. Use the toggle on the left-hand side labelled 'Show Individual T&M Items' to see every line item.