What do I do if LFG hasn't given me the money they owe me and won't let me speak to a supervisor?

I had a baby on Feb. 1st. I filed a claim with Lincoln Financial for Short Term Disability on Feb. 2nd. It was approved and I received a check about a week later. Problem is, the check was short by about $1,100. Last year my disability claim would have got me about $1,150 per week. This year it was upped to $1500 per week. However, they processed my claim at last year’s rate. They say that they are aware of it and that they still owe me almost $1,200 but say that because it’s the beginning of the year they are “busy” and the broker and National Account Manager have not updated my companies account yet. It’s now almost 8 weeks later and I still don’t have the money that they owe me. What should I do? When I call, they won’t let me speak to a supervisor, yet the girl I talk with typically calls me back – but just to tell me she has no updates yet.