We've often had trouble with the Dapol voyager. At one point it was returned to Dapol and came back running very well, for a while.... but then it reverted. A complete inability to negotiate pointwork was the problem - it would just derail at any opportunity.
I decided to put it back on the rails car by car, starting with the central power car. As I added each one, I checked that the bogies were free and the wheels had correct back-to-back settings. The power car, other central car and one front carriage went OK. But the other front carriage had a bogie problem: it was obvious that one axle wasn't properly staying in the phosphor bronze end caps.
Using a micrometer it was clear that the axles lengths were the same, but one bogie end was 0.5mm wider than the other. 0.5mm on an axle is a huge difference. Looking closely at the construction, the side frames seem to be some kind of pushfit into spigots on the central part, and one wasn't pushed in fully. A pair of long nose priers rectified that, and both ends then had the same width.
The train now seems to run OK round the railway. For a while it had pickup issues but after running for a few minutes it seemed to get past those. Hopefully it is fixed!