What can cause low testerone even when I get testerone injections every 2 weeks?

Different absorption. People absorb hormone injections at different rates and in different amounts. Patents with higher body fat absorb testosterone and remove it from the bloodstream. Fat cells(adipose tissue) transform (metabolizes) testosterone into estrogen. The liver disposes of the rest. The rates vary. This is why we need to follow hormones levels periodically. Doses need to be adjusted from time to time.
Incorrect siding. There are many forms of t replacement. T injections are almost the cheapest but require repeat injections every 14 days without fail. The dose you start with will affect your peak but not the frequency of injections. Consider switching to the daily gels or the three times a year pellets. Some men can benefit from the daily generic aromatase inhibitors.