If a slip isn’t being downloaded through Auto-fill my return it’s because the CRA has not received and/or processed it yet. Sometimes companies send slips to individuals weeks in advance of sending those same slips to the CRA.
Here is when slips are due to the CRA. Slips that arrive on time are usually processed in 1-2 weeks. You can see which of your slips the CRA has received/processed by logging into CRA My Account and clicking on “Tax information slips (T4 and more)”.
You aren’t required to use Auto-fill to do your tax return. If you have a copy of your slip, you can use the search box to find the relevant section and add it manually.