If your bobbin moves and doesnt return to its original position this means your lead has moved and your rig could be fouled so always recast as Yonny said.
Even if the bobbin raises with a beep or 2 I'd recast as you could be sat behind ineffective rods for hours on end, I try and recast every 3 to 4 hours anyway because fish pick up your bait quite a few times before 1 gets hooked so I'm always paranoid its sitting funny, especially with bottom bait rigs.
I have had a similar problem where my bobbins were lifting to the top but when I hit it nothing was there so I switched to a pop up and it stopped happening.