VM’s on Azure cloud are not being started fast enough to cover the user load. This delays launching sessions or failures to launch apps.
Support for Azure Resource Manager in XenApp & XenDesktop continues to improve and now features on demand provisioning of virtual machines. To take advantage of this new functionality it is required to recreate your Machine Catalogs. For more details please see: https://www.citrix.com/blogs/2017/09/01/provisioning-xendesktop-on-azure-just-got-a-lot-faster/