I need Strength Training Advice

Hi, folks, anyone has an advice on how to properly build up strength? I feel like I am stagnating to be honest, stuck with 3 (triceps, upright pushes) and 5s for about 2.5 month, with 8th for the rows/flies. Is it normal, or do you generally build up strength faster? I am asking because I see HUGE improvements in cardio endurance after only 3 weeks of jogging, but my strength, particulary arms plain sucks.

Will switching some of the Jillian's 30 Day Shred intervals in the morning to pure old fashioned strength training (say 2 days of intervals, 3 days of strength?) help? Or do I absolutely have to try those scary machines in the gym?