FIX Heartbeat message not sent

I am using FIX4.4 and QuickFIX/ v1.0.0 for its implementation.

I came across a situation where i had subscribed for Market Data and was successfully receiving Snapshot message then suddenly all communication with the Market Session stopped and i didn't receive any more snapshot from the server on the Market Data Session.

When i asked the support they said that they didn't receive any heartbeat from my end after sending the TEST Request messages. so they closed the connection. But some time we didn't see the TEST request in our log file even they sent out.

we configured heartbeat value is 45 secs.

My Questions are:

What might have caused the system to not send out a Heartbeat message?

Using QuickFIX/J how can i ensure my periodic heartbeat message?

Whats the best time interval for heartbeat? (Mine is currently set to 60 seconds)

For Processing Marketdata will take 10-15min if we don't have any session disconnection.

So if we increase heartbeat value, is there any issue?

we Processing marketdata for every 4 hour per day and during this marketdata processing only we are disconnecting sessions.