Our furnace has this issue where it will cycle but not ignite. There is no pilot, it is an electrical igniter. However, it does not do it all the time. In fact, when the cover is off, it does not seem to have any issues.
We run the blower 24/7. With the cover on, it will work then occasionally not work until you remove the cover and hit the reset.
I believe it is a Bryant Plus ?
I change the filters regularly due to the blower constantly running.
Find the wire coming from the pilot ignitor area tha goes to the electronic board or box.
This wire is usually orange or red.
The end at the box often gets loose and causes it to not spark like it should on start up.
It should spark,spark,spark several times and you can hear it snap each time.
It could be a weak ignitor, there could be too much distance between the ignitor tip and the metal(ground) that the spark jumps to.
But since you said it seems to be ok with the cover off, the cover could be touching the wire barely enough to move it and cause a loose connection.