This happens mainly because of two reasons. The first is that your pasta hasn’t been rolled thin enough – especially when making a homemade pasta recipe without machine! Evenly thinly rolled pasta is essential to help the entire piece (and collective group) to cook evenly. Otherwise, the outside will overcook while the inside is still chewy. The second reason is that the pasta isn’t cooking properly. You have to make sure to use a pot that is large enough to leave enough space for all the pieces to cook in boiling water.