BSA A7 gear box

[MCNA0055]

Hi I'm new to the forum and looking for any help/advise on an A7 gearbox. My dad's friend has a bsa a7 that needed a kickstarter spring so I removed the side casing swapped the spring and everything was going ok until I tried re fitting the side casing. The gear select a arm is in neutral but is stuck down making it impossible to locate the spring operated selecter in the side casing on to it and fit the casing properly. I have tried turning the engine over and manually moving the gear select a arm but it is solid it won't move. Is there something I am not doing right as I could select gear before I removed the side casing to fit the spring. Apologise for the long winded message but any help would be appreciated.