Most verbs gain the 'ge' in the perfect tense, but verbs with certain suffixes do not. I don't remember all of them now, maybe someone can find a list online? But they're typically words that have a suffix that is not separable, for example ver-, er-, be-, über-, and zer-.
The other class of verbs that do not gain the 'ge' are verbs that end in -ieren, for example 'telefonieren' and 'reservieren'. These simply drop the -en and gain a -t ending, i.e. Ich habe telefoniert, sie haben reserviert.
This should be helpful:
Weak Verbs Regular or WEAK verbs are predictable and can be "pushed around." Their past participles always end in -t and are basically the third person singular with ge- in front of it: spielen/gespielt, machen/gemacht, sagen/gesagt. The so-called -ieren verbs (fotografieren, reparieren, studieren, probieren, etc.) do not add ge- to their past participles: hat fotografiert.
Strong Verbs Irregular or STRONG verbs are unpredictable and cannot be "pushed around." They tell YOU what they're going to do. Their past participles end in -en and must be memorized: gehen/gegangen, sprechen/gesprochen. Although there are various patterns that their past participles follow, and they sometimes resemble similar patterns in English, it is best to simply memorize past participles such as gegessen, gesungen, geschrieben, or gefahren.
Mixed Verbs This third category is also rather unpredictable. As with the other irregular verbs, the participles for MIXED verbs need to be memorized. The mixed verbs, as their name implies, mix elements of the weak and strong verbs to form their past participles. While they end in -t like weak verbs, they have a stem change like strong verbs: bringen/gebracht, kennen/gekannt, wissen/gewußt.