Apart from general connectivity issues (such as limited coverage and network congestion) causing upload delays, certain events like frequent input switching (see example below) can cause delays in transmission.

A good example that often worries partners is experiencing delays when the External power (VExt) input quickly changes between multiple states within 2 minutes. For instance,

VExt is set LOW->HIGH - Data is send successfully to the servers/tracking platform.
A few seconds later.
VExt Input is set HIGH->LOW - Data is not sent immediately. Data is sent to the servers after 2 mins.

This behaviour is expected as Telcos/Networks do not want devices registering more than 30 times an hour. If a device tries to register more than 30 times in an hour, it will fail. 

This is due to the fact that the nrf9160 cellular modem throttles uploads to once every 2 minutes in order to gain network approval. Otherwise, you might experience something like this, device registers/uploads 30 times in 10 minutes, then fails uploads for the next 50 minutes which is not desirable.

In trip, a device attempts to register once and stays connected - all 1 connection/1 network registration. Thus, we can still get near real time updates and not updates every 2 minutes only.

To fix delays when monitoring fast input change (while not in trip), you can use "Stay Connected to LTE" Mode found in System Parameters -> Upload settings. Only set this if you know what you are doing; recommended to be used by advanced users only.