Pentasa and most of the other Crohns medications are maintenance drugs, which means they are designed to keep you out of a chronic inflammatory state long term. Most GIS willl seek to stop a flare and halt the inflammation initially with a short course (probably a matter of weeks) of prednisone, which is a potent steroid that attacks the inflammation. Then the maintenance drug will take over. I am a little surprised that your GI did not start you off with prednisone to allow the pentasa to begin holding you in remission. You might ask about that.
Pentasa is the lowest potency maintanence drug, and it does not always work for some people. The frustrating thing abuot Crohns is that every case is different and every patient responds differently. So it is really trial and error to see what works for each person. The next step up from Pentasa would probably be 6mp or imuran, which are similar drugs that turn down your immune system a notch or two to control the inflammatory response. Above them are the biologics like remicade or humira. Traditionally, most doctors have started at the bottom and worked up through the drugs from pentase to 6 mp an remicade at the top, on the theory that there is no need to prescribe the most potent drugs until you know the patient will not respond to the lower ones. Some are now starting farther up the drug chain. Yours seems to be more traditional. I'm not sure there is a lot of research to support either approach conclusively.
As for strictures or blockages, the classic symptoms are severe pain, bloating and vomiting. It's usually bad enough to send you to the ER. A CT scan wil usually show it. From your post I don't sense that you are there now . . . and be aware that not all Crohns patients experience strictures or obstructions. You said you were recently diagnosed, and that would imply you had a colonoscopy. It would have shown any narrowing in the colon. To look for narrowing and potential stricture areas in the small intestine, they will do a small bowel follow through series of Xrays after you drink barium. It is a pretty conclusive test too. A good rule with this disease is not to borrow trouble before it appears, as some patients have serious complications early on (like strictures or fistulas), some do not have them for some time and others never have them at all.
I would think your first step could be consulting with the GI about your ongoing symptoms, noting that the Pentasa is not oding the job at this point and asking for a short course of Prednisone to get you into remission. Then perhaps continue with the penatasa unless and until severe symptoms return and asking for a step up, perhaps to 6mp, with maybe a second course of prednisone to stablize you.
There are some good general informational links at the top of this forum page and also on the Crohns Colitis Foundation of American website. Good luck . . . you are asking the right questions.