Check the diagnostic logs on the Jetpack for more information. Disconnections can occur either from the VZW network or locally over WiFi so you will have to narrow it down to one area or another.
If its a WiFi disconnection then your top two suspects are distance and interference.
To minimize distance keep the MiFi directly next to the device in question.
To minimize interference scan the air for competing wireless channels over your current technology. Then switch to an open and non overlapping wireless channel for a better chance of success. Channel decisions are based on either the 2.4 GHz or 5.0 GHz spectrum so you will need to confirm which one you are using first.