As the chip is very small and is mounted directly under the gold coloured contact the tolerances are very tight and each contact has to tocuh the correct part (power, data, etc) otherwise it'll either not power up the SIM or the SIM will not exchange data with the device so it won't log on.
Dropped an old Ericson I had a couple of times and although the SIM moved only slightly the phone was no longer exhcnaging data with it and so the phone lost connection with the network until I removed and reinserted it.
Virgin (ADSL) => Namesco => Newnet => O2 => Plusnet => Zen => Newnet => Zen => Freeola => Vivaciti (using O2 Wholesale DSL) => Xilo (C&W Wholesale) => Xilo (O2 Wholesale) => Xilo (TT Wholesale due to O2 Wholesale closure) => Zen LLU
Router: Billion 7800N
Note: I don't lay turf for anyone. astro or otherwise, all views and opinions expressed are my own based on experience.