It sounds like you're building a new culture, so pushback is set to happen no matter what. I'd say to stick to your guns and keep working towards it and soon your band will practice easily.

One clinic I went to at TMEA said to always be a cheerleader for practicing. Mention it all the time everyday and eventually it'll get in their heads.

Maybe also put the ball in the kids' court? If they're busy with this and that, ask them what time would work best for them and if they can come to the band hall to do it. That'll also show you being accommodating and giving them a chance to schedule a time.