Just guessing but if nothing else has changed then only thing that can increase the vacuum is sealing of the system. Possibly sap drying inside very minor leaks causing them to seal up?
2023 - 130 taps, 90L from 4,000L as of mid March
2021 - 84 taps, 50L from 2100L
2020 - 100 taps on buckets, 21L syrup from 2700L so far (FEB 26-Mar 13) and then the pandemic hit! End of our season!
2019 - 62 taps on buckets, 95L syrop from 3215L sap
2018 - 62 taps, collecting by hand, 90L syrop from 3200L sap
2017 - Lapierre Waterloo Small mini pro with 40 taps
2014 - 2016 40 taps making one or two batches on a 2x6 flat pan over an open arch as it would have been done in 1900