Hey, players do not pop in skills every week! Are you getting them 48 minutes per week? A lot determines how often they will pop. Trainer level, how many positions you are training etc, and of course player age! Skills like Defense tend to be the slowest skills to train, at 18 depending on height you would be looking at a pop every 2/3 weeks or so. Doing skills alternately will not wield worse results but if you enjoy seeing pops i would train 1 skill to a specific level then another, or pops will appear to be super slow doing 1 week of each.
If they are popping every few weeks i'd say you appear to be training correctly, but as i've said above you'll have to check you are meeting all the specifics to wield best training results.