The good thing about being a Space Marine, is that you can fight pretty effectively with whatever is to hand.
Yes, there are the traditional weapons (Bolters of varying size, Chainswords etc), and Codex Recommended equipment. But there’s still reality to contend with.
For historical context? In Rogue Trader they might be armed with Autoguns, Shuriken Catapults etc. And in-game history, the Blackshields of the Heresy era likewise used pretty much whatever they could get their hands on.
Whilst I don’t think many Chapters would opt to go without support weapons in a squad regularly? They can be a funny lot, Space Marines.
Perhaps they have a non-standard process for blooding recruits. So once you’re out of the Scout Company and are a full battle brother, you join a basic Tactical Squad. One armed only with Bolters. Or perhaps it’s a mark of Veterancy within a Battle Compant. A sign the Brothers in the squad are so competent, they don’t need the support, or are sent on rapid strikes to clear out buildings where, outside of a flamer, the other weapons are just an encumbrance. Or as a way to honour the Crusade Legions, where the Tactical Squad was just Bolters.
Perhaps your Chapter is down on its luck, and has had to prioritise its supply chain to keep the basics flowing.
Various and sundry other possibilities exist