You'll see what looks like the head of a nicely poured Guinness forming on top in the oil reservoir. If you are sucking air from one of the lines the pumps will compress the air and not the oil. When the machine sits for a while the air in the line dissipates and you are good to go again until too mush air is sucked in. The filter could cause this issue by restricting the oil and the pumps suck in air because it's easier.
Hope this helps,