Thank you for posting!
An important note up front, the network infrastructure configuration you describe in your older building is not supported or recommended by Plugable.
In addition, there are limitations and potential network issues that can arise from only using two pairs in an Ethernet run.
The first is that doing so will limit the connection to a maximum of 100Mb speed (also known as 100BASE-TX -> https://en.wikipedia.org/wiki/Fast_Ethernet#100BASE-TX ). Gigabit Ethernet (also known as 1000BASE-T -> https://en.wikipedia.org/wiki/Gigabit_Ethernet#1000BASE-T ) requires the use of all four pairs, or eight individual conductors.
The second is that if the two pair configuration was used to ‘double-up’ the amount of network drops that could be run using a single cable (one physical cable with eight conductors with two pairs terminated to two separate RJ-45 jacks) there is the potential for problems.
While the configuration you describe is not supported, based on your original description, “our older building used 2-pair (pair 1 & 2)” there are two things to double-check that could possibly help.
The first is that the network cable used meets or exceeds the CAT5 specification. Lower grade cables (such as CAT3) will not work at 100Mb.
The second is that pairs #2 and #3 are being used -> https://en.wikipedia.org/wiki/TIA/EIA-568#T568A_and_T568B_termination Those pairs are used for 100BASE-TX, not pairs 1 & 2 as you describe.