First thought: Not all 9mm pistols will cycle reliably with SWC bullets. My Browning Hi-Power does well, but several other have been problematic.
Second thought: Get out your calipers or micrometer (or borrow one) and measure the diameter as sized. Depending on the hardness of the lead alloy used in casting the sized diameter may not be .356" as expected. I shoot a lot of home-made bullets in .38 and .357 revolvers made of pretty hard alloy, and after running through a .357" sizing die the finished diameter is .3584" pretty consistently. Not all sizing dies are exact in finished diameter, and every lead alloy will show varying degrees of "spring-back" after sizing.
Very true. Cast SWC bullets can be accurate and feed reliably, but this all takes a good bit of experimentation. There is little need to "slug" a bore unless you want to. I've found shooting the largest diameter bullet that will allow easy chambering works best in all semi-automatic handguns.
Some brass may be tighter than others and size a bullet smaller when the bullet is seated. Mixed brass often works okay, but I wouldn't recommend it if you have an interest in accuracy. I've had good results with older Winchester brass that I've used for a long time.
There's not a lot of 9mm load data for 150 - 160 gr. cast SWCs and one can easily get in trouble with powder charges and seating depth. While such bullets can be used, I've seen this as more of a "what can be done" stunt rather than something of practical value, but that's only an opinion based on my perspective.
The best cast 9mm bullet I've found to date has not been a SWC but an obsolete Lyman .38 Special design, #358212, a roundnose 150 grain.