Kneeling down? is this on the floor? is your mat on the floor? that may be the problem. Try working on the kitchen counter - a great height - or at least table height.
Then check your mat... they can sometimes be a problem if you cut in the same place(s) over and over... the invisible grooves made with the blade become clogged! They are self mending but.... and by all means, use the green Olfa ones, largest you can reasonably afford and store... the other plastic jobs are awful, in my opinion.
last... make sure you are using 1 blade... sometimes two can even look like one and that causes a problem... ask me how I know! HAhahaha
Good Luck.