We just released a new patch which fixes an important federation problem. Due to a mistake in the code, activities were only sent out once, even if delivery failed (e.g. because the target instance is down, or a temporary connection issue). Instance admins should definitely upgrade.
This problem is fixed, so that failed deliveries are retried up to 9 times, with exponentionally increasing intervals. The only limitation now is that it will not keep retrying after Lemmy is restarted, but that case should be less common.