extrusion

hi I recently finished building a 3d printer which uses ramps1.4 but my plastic extrusion seems to have a problem. during the first few second of the print the plastic extrudes fine but later it gets jammed in the hot end and doesn't allow the extruder to push the plastic.

the bed level is adjusted so that it doesn't block the nozzle.
I don't know much about the extrusion multiplier setting but I changed the value from 1 to 0.9 and it didn't make any difference

I use a 1.8° stepper motor and an mk8 extruder

Is your hot end fan turning? Is it pushing air in the right direction? Usually there is a small arrow on the fan that shows air direction.

omg yes my fan wasnt working thanks allot