Personally when I play tank without hard initiation (volibear, allistar, xin) I save my gapcloser until I'm quite close (almost blindside) If they see you coming they will kite you and start to fall back will which cause you to separate from your teammates leaving you on a very bad dilemma. At least on low elos, just show yourself and wait for someone to make a mistake and get a tad to close in order to poke you and then Q him/her into your team while you zone out the other members. For volibear specifically, if I don't caught anyone 1-1.5 sec after activating my Q I stop chasing wait for cd and try again becuase chances are that even if I caught someone it will be a bad engage.
Flanks can also work but make sure you are facing the right direction when you fling.... I have saved quite a few oponents by chasing the wrong way.
Use your slow (E) as an additional gapcloser, if the enemy has no dash once the slow hits the fling will succed.
Flash+E+Q+R.... Downside is the flash heavy cd. Ghost can be used on a less effective way but the cd is a bit more bearable.
Swiftness boots would also help as you depend on your movement speed alone to caught opponents.
If everything else fails run Exhaust and exhaust them at max range (the slowing smite can also be used for the same purpose but is not as effective as the exhaust)
now... Unless you are uncertain if your team/follow up can kill whoever you threw to them don't stick to it. Position yourself closer to your next target zoning out possible treats to protect your team. Really once they were fling to your team your job with that particular target is pretty much done... as a tank your job is not to focus the damage on the carries or a specific target (If you can without afffecting your role is a plus but is not your priority) Your job is to draw fire at you... if you caugth the adc (flung) then walk up over the mage/assasin to discourage him from jumping in in your squishies. If you caught the support/the top walk over the adc and prevents him to get in range to shot anyone but you. You don't have to be right in the face but keep them at bay.
the active on this item will give you more movement speed so you can get close to the enemy. If this doesn't solve your problem, you're going to have to get more creative about your engages like picking up scanner lens and a pink to clear out vision in an area and ambushing an unsuspecting enemy.
