Panama Red...
Good comments from Qshake and ishvaaag. Both are relevant to the loss of slump from pumping.
The most common reason for slump loss from pumping is that the aggregates were not in a saturated, surface dry (SSD) condition when the concrete was batched. This results from lack of attention to the stockpile moisture as noted in ishvaaag's comments. Most coarse aggregates can absorb between about 1-1/2 and 5 percent moisture by weight. Most fine aggregates at about 1 percent or less. The pressure caused by the pumping action forces the water added at the time of batching into the deficiently saturated aggregate, thus removing some of the mix water available for "viscosity" of the mix, resulting in a slump loss. This condition is exacerbated by aggregates that are excessively porous (limerock and lightweight aggregates, particularly). A second common reason for slump loss is the reduction of entrained or entrapped air in the mix resulting from pumping action. There are several other, less prominent reasons for slump loss, all of which can be controlled.
Qshake makes an excellent point about taking slump tests at two locations. For legal purposes, the slump must be taken at the discharge chute of the transit mixer (concrete truck). The reason for this is that this is where the ready-mix producer's delivery specification responsibility ends, so you must assess if you got what you paid for. Taking an additional slump test at the end of the pump hose is also advisable, since this is the slump you have to work with (though it is not the slump that was specified!!) Further, there are several ways to compensate for slump loss at the end of the hose, the worst of which is to simply add water!
Your 1/2-inch slump loss is pretty good! In my area of the US, we would be pleased with this result as we have several of the reasons for slump loss which tend to compound on us, particularly in the summer (aggregate saturation, air loss, hot weather!)