There are a couple of possible reasons why your Twitter account is not displaying some mentions in the timeline: 

1. The Quality filter option is enabled
2. One or more options selected in the Muted notification filter

Disabling the above-mentioned options solves the problem. 

Twitter Quality filter

A common reason for missing mentions is the Quality filter option. Once enabled, this option filters mentions on Twitter’s side before they reach the client app.

To check if the Quality filter option is enabled for your account, follow the steps given below:

1. Log into Twitter using the web interface or an official Twitter app. 
2. Click the More button on Twitter’s web interface and navigate to Settings and privacy > Notifications > Filters:
   
   
   
   
3. In case of the Twitter app, tap your profile icon > Settings and privacy > Quality filter:
   
4. Uncheck or toggle OFF the Quality filter option.

Note: Clear the Twitter cache in a web browser and mobile device to reload all the mentions from Twitter.

Muted notification filter

Another possible reason for missing mentions is one of the options selected within the Muted notification filter. The muted notification contains filtering options that mute you from mentions of people who you don’t follow, people you don’t follow, people with pending account verification, and so on.

To check if the Muted notification filter has some enabled options, follow the steps given below:

1. Click the More button on Twitter’s web interface and navigate to Settings and privacy > Notifications > Filters,
2. Click the Muted notification filter:
   
   
   
3. Uncheck all the options.