Why is it harder to fill at the mid-point with my new broker?

Getting a fill at the midpoint is a function of there being a counterparty willing to transact at that price whether it be the market maker or another trader. That has nothing to do with who your broker is. One broker can't give you a better price than another - they're just submitting orders to the option exchanges.

I have found that in some stocks whose options that I trade, the market makers split the bid more often than not. And in others, they won't even budge a nickel on a 25 cent or wider B/A spread. Chances are, you're either trading options with lower liquidity now or you're involved in some of these "won't budge" options.