Understanding Society User Support

Hi, I'm interested in including father's social class in my analysis but I've noticed that there are a large amount of missing data for this variable. I looked at a previous response on this forum concerning this and it advised using the cross-wave data (xwavedat). However, there are a large amount of missing data here too - for example if I merge the cross wave data into Wave R, 40% of respondents show as "not applicable". I've also tried adding in the data from all of the previous waves, but this left a similar number of 'NAs'.

During this process, I noticed that the questions about father's social class were not asked in waves B-G. I thought that maybe the reason for the large number of missings is that people who joined the sample between waves B-G were never asked the question in subsequent waves. Is this the case?