What features do these mobile TS have? (2)

For example, on many DSPs, your daily budget will also throttle the number of impressions you get. If you're bidding on a limited pool of traffic, that can often throttle your campaign to the point that you spend a lot less than your daily budget every day, as the DSP's algorithms get their predictions a bit wrong. Try doubling or tripling your daily budget and watch the increase in impressions - sometimes you'll need to have a daily budget of $100 to spend $33/day, for example.

On some platforms, available funds will also throttle your campaigns. In particular, if your available funds are very low but not zero (less than $100), that can result in your available impressions slowing to a crawl.

Finally, do check if the platform you're on has any restrictions on content. On some DSPs, for example, some kinds of ad will be allocated only "non-brand" or low-quality traffic. Ask the platform's support team if this is the case, or ask on STM. If it is the case, finding a way to get your campaign out of the low-quality bracket will often open up an enormous amount of additional, lower-competition/higher-quality inventory.