This may have several causes,
1) A too small capacity/tank of the hotmelt unit
2) The achieved temperature or the temperature setting is too low (refer to the hot melt technical data and measure the actual temperature)
3) The type of hot melt does not melt easily (it has a high Melt Flow Index, MFI)
4) The glue level in the tank has been too low.
5) The hot melt equipment is not properly connected
6) The hot melt is blocked somewhere in the system, e.g. by burned hot melt.