This almost always traces back to the Calculation Rule under Settings → Sessions → Credit Settings (9.2), which has two very different modes:

  • Session Count (fixed) — every session costs exactly 1 credit, no matter how long it runs.
  • Duration Based (variable) — credits scale proportionally against a "1 credit = N minutes" baseline you set, so a session that's shorter or longer than your standard length deducts a fraction or a multiple of a credit automatically.

If your institute is on Duration Based and a 30-minute session only deducted 0.5 credits when you expected 1, that's not a bug — it means your baseline is set to 1 credit = 1 hour. Set Credits per hour to 2 instead if you want a 30-minute session to cost a full credit.