You have hit a security pin inside the lock. The Iseo 5 pin Euro profile locks have 3 spool pins inside.
To beat them either use less force to turn the plug and keep reducing it untill the spool pins stop catching, or use what is know as 'negative plug rotation' To do this pick the lock and let it turn a bit like you said, then keep it in that position. Insert your pick into the lock and feel for pins which feel totaly jammed - these are the spool pins that have hung up on the sheer line. Reduce tension a lot and press down on the closest pin which feels totaly jammed, and wait untill you feel it set straight and then target the other security pins to set those. When you press down on a security pin that has false-set you will feel the plug move counter-clockwise, this is normal and is because the spool pin because of it's shape is trying yo turn the plug back to the neutral position. Let the plug turn a bit but don't let it turn to much or the lock will reset.
Iseo are very easy to pick, and respond well to negative plug rotation.
Hope this helps.
