sounds like you have done all the right things. It may be that the fermenter isnt airtight and the CO2 is sneaking out. You can test this by gently depressing the lid or the sides and see if the airlock moves, and holds a constant pressure. If the levels initially move but then go level again you have a leak - and that should be fine for a week or two of primary fermentation.
you can also test a sample with a hyrometer of course - but you would need to have taken an initial reading when you started. lots of info on that on here.
you can also test a sample with a hyrometer of course - but you would need to have taken an initial reading when you started. lots of info on that on here.