the problem description hints at a missing IGMP querier.
On EXOS, the querier is configured on a VLAN that has an IP address using the following command:
enable igmp vlan VLAN_NAME IGMP_VERSION This enables IGMP queries in this VLAN, allowing IP multicast to work correctly inside this VLAN.
IGMP snooping, enabled by default, requires either bidirectional multicast traffic or regular IGMP messages to keep the cache entries current. The relevant messages are created when an end system joins the group, thus multicast initially works, but this time out if there is no querier in the VLAN to trigger recurring group membership messages from the end systems.
To transport multicast from one VLAN to another needs additional mechanisms, e.g. multicast routing using PIM.