You can use volume indicators for long term and if it is not working for you, you can either read a bit about it or start day trading.
There is nothing that you can do and others can’t in the forex market. It is all in the books that traders like you and me read.
I assume you are trading stocks, has the average daily volume reduced in the past two weeks for the stocks that you have trouble being filled? Perhaps it's more likely that your strategy has been independently discovered by someone else. You could also try two different brokers (that are not owned by the same parent company or group), to see if that has any difference. I'm not 100% sure if I'm right on this, but it seems that on IBKR (as an example) there can be different exchanges providing the bid and the ask prices, so different brokers might have access to different exchanges.