It is bugged sometimes, but also many people have no idea on how it works.
When the message is 'you can go to pick it up' the weed is NOT on the weed field.
There is a red marker appearing where the weed has grown. Sometimes you are out of range, or you can not see it due to other blips on the radar.
When you arrive to the red marker, the weed will be added to your inventory.
The bug is that in some cases you might arrive at the marker, and weed will still not be grown.
This is going to be fixed in RS4.