squats and the lower back

it seems now that as i get heavier on the squats and my lower back gets somewhat stronger, its never up to par with my legs, when ppl spot me or look from the side they say it all seems good except when i come back up from the bottom i dip abitand my legs go up then the back muscles catch on and i bring myself up, this has been goin on for awhile now it does’nt hurt or anything only the sacrum gets crazy sore but after a few days recovers fine. I went to see physio about it and he said its fine just focus on the technique. This is hindering my progression i think as i can’t do the lift smoothly, i do RDL and back work, they are strong, but the back just does’nt catch on. Any other alternatives like should i lower the bar alittle lower on my traps or keep it high?