This is not a limitation on the MX side.
This most frequently occurs due to either bugs or limitations in the router the users are sitting behind. You could try upgrading the firmware on that device.
The best solution is to buy Cisco AnyConnect licences, and change over to using that. It is more reliable and superiod in every way.
https://documentation.meraki.com/MX/Client_VPN/AnyConnect_on_the_MX_Appliance
AnyConnect licencing is honesty based, so you could give it a little test before committing the money. It's realtively cheap though.