Sometimes you’ll get a disappointing session, there’s not much you can do about that other than minimising the times it might occur by trying to not PE when you are tired and making sure your general health is good.
Jelqing can also get boring. Sometimes porn helps and sometimes it hinders.
In regard to spots, it varies some guys think that unless a session produces pin prick spots they are not working hard enough others manage to gain fine without getting into spot territory. The spots are an effect on the skin and in the end the skin is not what you need most work to stretch, so either way it may not be a good indicator either of overwork or of working enough. You will need to work out when you’ve had a good session by how large you are afterwards, how long the extra hang stays after a session and the tiredness of the tissues (the feeling of being worked). Once you know what is good for you, you’ll be able to repeat it.
A few pin prick spots is definitely not a reason to skip a session - they should be gone the next day. If you get bigger spots, maybe 1/16” up you are starting to force blood out of vessels rather than capillaries and this is a problem.
Hope that helps.