Welcome to CS (first time Ive said that - I almost feel like an oldie!) 8-)
My (not so experienced) advice would be to problem solve by taking one thing away at a time. If you tighten the handle a bit more and it stops, that is what your problem is, if you dose your portafilter a little less and it stops, that is what your problem is, etc.
Ive had this a number of times, at all the above times at one stage or another (ie; overdosing, not tightening enough, etc) and it was usually user error.
Or it could be that your seal in the group head is on the way out and might need replacing. They (the good sorts on this site) recommend that you dont leave the portafilter locked in for extended periods (Ive taken to removing mine overnight once Ive made my last coffee at about 6.00pm and given it a thorough clean), and then left it out until I go to make my morning coffee at which stage I plonk it in when I first switch on the machine so it has time to warm the portafilter when I go to make my work takeaway). I also get into the seal area when Im doing my thorough clean each night and brush away all the lose grounds around the seal with a soft paint brush.
Hope this helps.