What Highlander says is true for the 50cm. One of the reasons this joint can be hard to make completely is that the vinyl of the abdomen gets pushed into this joint and prevents the two parts from mating completely.
Here's a look at the two parts at the leg/hip joint without the vinyl. You can see there is a circular groove around the peg of the waist part. The tube of the hip part needs to fit cleanly and completely into this groove in order for the joint to be made firm.
Frequently the abdomen vinyl is trimmed very tightly around the opening.
With the vinyl so close it is common to pull the edge of the vinyl in between the two parts as you push the two parts together. When this happens, the two parts do not mate completely and and if a big enough piece of vinyl is pulled in, the additional pressure can crack the outside ring of the waist part and/or the tube wall of the hip joint. The joint may seem firm at first but after a move or two the vinyl crushes and/or the hip joint part cracks and the unlocked leg becomes "swingy".
A preventative measure for this is to take a sharp xacto knife and trim the vinyl of the abdomen around this hole. The hole can be easily enlarged and is hidden by the leg parts so you don't have to worry about making the cut cosmetically perfect.
As for the shoulder joint, as Highlander stated, it is very easy to miss the receiving tube of the shoulder part.
With all the adjustability of the 50cm shoulder part it is easy for the receiving tubes to become misaligned with the hole in the chest vinyl. It would be a worth removing the chest vinyl, "zeroing out" the articulations of the chest part and inspecting it for damage. It is not uncommon for the receiving tubes on the chest part to crack. These cracks can be very fine and hard to see. Inserting the shoulder part onto the tube can help make any cracks appear. These cracks can be repaired by applying super glue to the crack and clamping. If the cracks are too large replacement will be necessary.