Blades of grass can be too long and too flaccid to stand upright so that they cannot present themselves to the bottom blade for cutting. This is not uncommon when first introducing a cylinder mower, and you need to get the lawn to a position where this doesn’t happen.
Just to make sure this is the case, pick one of the long blades of grass which are lying down. Are there any cutting blade witness marks on that blade of grass (dark green mark where the cutting-blades have squeezed and not cut the leaf)? If this is the case, then the blades need adjusting. If the uncut leaves are still lying down, they are slipping under the bottom blade and not getting cut. The main reason for this happening is that the length and weight of the leaf make it lie down (think of long hair vs a crew cut and how the hair lies or stands up- if you pinch-off or cut a long blade of grass to the intended cut length, you should see that the remaining leaf stands up as we want it). This is about how regularly you are cutting the grass. Although a cylinder mower is the most precise way to cut a blade of grass, it is best at cutting ‘short grass shorter’. Using the front wheel kit (part number: QCSWK for QC cartridge mowers) is one way to get the grass down to a sustainable height, but without the front roller, you will not achieve the best stripes.
There is a basic rule which should not be broken to achieve a great lawn. It’s about not cutting more than 1/3rd of the height in one go. If you look at the long uncut blades, are you trying to remove more than 1/3rd of the leaf length? This will not give a great lawn in itself and your cylinder mower will struggle to give the clean-cut that they are renowned for. So first we need to get the grass cut to the target. This can be done by mowing in a few directions with the front wheel kit on. Or use a rotary mower. Then pay more attention to the frequency of cut, so that the longest grass doesn’t get too long and avoid the cutting blades. Use the scarifier cartridge regularly (up-to weekly during the growing season) to encourage upright growth. Better nutrient levels also make the plants stand more upright and better able to recover from scarifying. A denser sward (ie a higher plant population per square metre) also helps.