If this is not your first roll of 35 mm HP5+ then you know it has a base density of close to one stop. If it's much higher than that, it may be due to the long development time. There's not much use trying to overdevelop HP5+ in Rodinal 1+50 to pick up the contrast. A gradient of about 0.65 is about the best you can do. After that, it does what you saw: builds up fog which actually may reduce contrast.