What do I do to fight for my raise that I was supposed to get while on maternity leave?

May 22, 2015 - I went on disability due to experiencing pain being pregnant. July 9, 2015 - My daughter was born. July 15, 2015 - My raise was due. I was off work until November 6, 2015 due to a fractured tailbone resulting from pregnancy. When I returned, I was still at the $16.66 per hour pay rate and not the $17.56/hour amount I was supposed to be at. The manager signed a new contract and everyone was placed at a new amount based on time served, but I only went to $18.00/hour instead of the $19.50/hour I was supposed to. Now, the next increase will be in April where it will again go up $1.50/ hour but again I will be behind. I need help in getting this taken care of. I contacted our HR department, but they said that it was accurate because they went off of what I was making when the contract started (November 23). I know that it isn't right, but I need help with this.