1) The first thing to check is that the silo is gas-tight, ie that it meets at least a three minute, half-life pressure test. (Check before filling and when full as the weight of the grain can open gaps around the bottom slide plate.)
2) Check the application rate is enough to treat the entire storage volume.
3) Check the exposure period is long enough according to the grain temperature.
4) Get an expert to ensure you’ve correctly identified the insect pest, particularly if you suspect it’s the flat grain beetle, cryptolestes spp.
For more information see ‘Fumigating with phosphine, other fumigants and controlled atmospheres’