Please refer the following link to see if there is any error in Intune portal App troubleshooting details.
https://learn.microsoft.com/en-us/mem/intune/apps/troubleshoot-app-install#app-troubleshooting-details
Also, please ensure that the user is listed as Enrolled by User in Intune Device Overview pane. If it shows NA, then deploy a config policy for the Intune Company Portal. For more information, see Configure the Company Portal app.
In addition, for the Intune service to successfully deploy a new IPA file to the device, you must increment the CFBundleVersion string in the Info.plist file in your IPA package.
https://learn.microsoft.com/en-us/mem/intune/apps/lob-apps-ios#step-5-update-a-line-of-business-app
If all the prerequisites have met, to find the root cause effectively, we may need to collect some logs or see the status from Intune backend. It is recommend to create an online support ticket. Here is the link:
https://learn.microsoft.com/en-us/intune/get-support#help-and-support-experience