Can you please tell me if you are using SA-MP 0.3.7 R4? I think Kye has broken something in regards to pickups. They fall through the fucking floor..
To clarify I believe there are two issues in one here. The first issue is that pickups appear but cannot be collected. I'm pretty sure this is a script bug but I haven't been able to replicate this issue in the sterile test server environment.
The second issue is a pickup not appearing at all. They sometimes fall through the ground. The two situations I have observed this in are in interiors, where the pickup falls through the floor down to the ground below (1000 meters down), and the next issue is when it falls through an object to water below, such as on a bridge over water.
The second issue I can do absolutely nothing about as it's a SA-MP issue. I don't control the pickups, I just tell the server where to place them and it tells me when someone picks it up. I have no control past that.
EDIT: Ok so it turns out my script was getting the 'true ground position' to create the pickup on. In interiors the height of the ground will be down below. This is the issue. I have removed this code so it now just creates a weapon pickup in front of you wherever you are no need to get the ground position. This will be fixed in 1.4.5. Not sure about issue #1 though (being unable to collect a dropped weapon). Will have to do some more testing for that one.