Your derm should've have prescribed you 200mg doxycycline (100mg every 12 hours). If you don't see noticeable result in 6 to 8 weeks, it's either minocycline/bactrim or accutane. Remember after 12 weeks your derm suppose to end the antibiotic treatment to prevent bacterial resistant.

What time period did your doctor prescribe for? My understanding is that low dose tetracycline antibiotics are prescribed more for their anti-inflammatory effects than for antibiotic effect. Doctors here prescribe them for 3-6 months. It can take a couple of months to see much effect so I would suggest perservering a bit longer yet. Take some pictures to be able to compare your progress from now to each month end. Have a follow up appointment with your doctor after 2-3 months.