Tag size
Large PIT tags have better read range than small ones.
Tag quality
There are many manufacturers that produce PIT tags but not all tags have the same performance. Another measure of quality is tag-to-tag consistency from a manufacturer.
Reader type
A proximity reader measures tags within a few centimeters of the device. They are a battery powered handheld device that is used to scan for tags at close range . A long range reader has more power and scans continuously using large antennas.
Antenna
The size and efficiency of the antenna can have an effect on read range. For a given tag and reader, the read range can be quite different depending on the antenna used. Some antenna designs will also work with a particular tag better than others.
Orientation
Read range will be less when the tag is at angles and positions other than the optimum orientation.
Proximity of metal
Metal near the antenna can bock or weaken oscillating magnetic fields.
Noise
Electrical signals in the same frequency range can interfere with the tag signals.