Plants with no fruit but thriving leaves?

Could be a bad season or just bugs.

caffeine - probably no
nutrients - possibly  There are three major nutrients (nitrate, phosphate, potassium) that plants need in varying amounts.  Nitrate promotes foliar growth, while phosphate promotes flowering and the set of fruit as well as stem strength and root growth.  Potassium is for overall health and growth.  So if you were using a fertilizer with a high nitrogen content but low phosphate, you might get the situation you're describing.  You can get an idea of the relative strength and amounts of each nutrient by looking for the NPK ratio on the bag/box/bottle of fertilizer you've been using.  This should be on the front label in bold, and will be three consecutive numbers such as 5-5-5, 10-10-10, or 5-10-5. to show the percent amounts (in the order of N-P-K).  The first and second I gave as examples have the same ratio of nutrients (all the numbers are the same relative to each other), but the second is twice as strong (the numbers are double the ones from the first).  The third has the same amount of nitrate and potassium as the first, but double the phosphate (the second number is double the others).  To promote flowering, you would want to have a higher middle number.  WHat each plant needs as far as nutrients depends partly on the richness of your soil, and partly on the species of plants you're growing.  This is something you can look up online.

It's also possible that neither are to blame.  Some plants (ferns, for example) reproduce by spores, and never produce flowers.  Other flowering plants set their flowers in response to temperature or the photoperiod (the # of hours of darkness compared to the # of hours of light they get like a poinsettia).  Also, the intensity of the light can make a difference - just look at plants growing in a field, and the same plants growing several feet into a wooded area where the trees shade them - which may apply if you're growing the plants under artificial lights or near a window only using natural light facing away from the direction of the sun (north-facing window in the northern hemisphere in winter) or with sheer curtains or an awning reducing the amount of light.